e.coli nusa rna
TRANSCRIPT
A Stmcture/Function Analysis of the Interaction of
the E.coli NusA protein with RNA polymerase, the
phage h N protein, and nut site RNA
Thien-Fah Mah
A thesis submitted in canformity with the requirements for the degree of Doctor of Philosophy
Graduate Department of Molecular and Medical Genetics University of Toronto
O Copyright by Thien-Fah Mah 1999
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A Structure/Function Analysis of the Interaction of the E.coli NusA protein with
RNA polymerase, the phage h N pmtein. and nut site RNA
by Thien-Fah Mah
Department of MolecuIar and Medicai Genetics in the University of Toronto
Submitted for a Doctor of Philosophy degree, 1999
Abstract
NusA is an E-coli protein that controls transcription elongation. terminrition and
mtitermination. In this thesis. 1 show that the functions of NusA in transcription are
fxilitated by interactions with RNA polymerase, RNA and the h N protein. Use of a
series of deletion constructs of NusA ailowed me to identify specific regions of NusA
involved in specific interactions and in various aspects of NusA function.
Genetic evidence suggested that NusA may interact with the box4 portion of the
N-utilization site (nut site = ho-rA, interbox, and boxB). By constructing multiple
nucleotide siibstitutions in the nut site, 1 showed that the identities of certain nucleotides
lit the 3' end of bosA and in the interbox were important for NusA to associatc with an N-
u t l r site cornplex. NusA association with RNA in the presence of N is presurnably
fxilitatcd by its S I and KH homology regions, two types of RNA-binding domains in
NusA. Elimination o r mutation of the S 1 homology region prevented the association of
XusA ~vi th an N-ntrt site cornplex.
Using affinity chromatognphy experirnents. 1 found that RNA polymense bound
equally well to an amino-terminal RNA polymerase-binding region in amino acids 1-137
and a carboxy-terminal RNA polymerase-binding region in arnino acids 232495 of
NusA. By contrast. the a subunit of RNA polymerase only bound to the carboxy-
terminal RNA polymerase-binding region of NusA. N protein also bound to ri carboxy-
terminal region of NusA. and both N and a allowed NusA to associate with RNA in a gel
mobility shift assay. When the carboxy-terminal region of NusA was deleted in NusA
( 1-348). the loss of N-binding and a-binding ability did not abolish NusA function in
termination and antitemination assays. This minimal functional NusA protein retained
the KH and S 1 homology regions and the amino-terminal RNA polymerase-binding
rcsion. Unlike full length NusA (1-495). NusA (1-416) could bind RNA on its own.
These observations suggest thclt the carboxy-terminal region of NusA inhibits RiVA
binding and that this inhibition c m be relieved by interaction with the h N protrin or the
cc subunit of RNA polymerase.
Table of Contents
Thesis Abstract
Table of Contents
Ac knowledgements
List of Figures
List of Tables
Chapter 1: Intrduction
Transcription in E.co/i
Initiation
Elongation
Termination
Rho-independent temination
Rho dependent termination
Regulation of Elongation by the Bacteriophage h N protein
The nrrt site
The Processive Antitermination Cornpiex
N protein
Nus factors
Mechanism of Antiterrnination
Other Antitermination Systems
ii
iv
vii
viii
X
11
11
12
15
23
24
26
28
30
33
33
35
45
46
Q-mediated Antitermination 46
rrn Antitermination 48
pr~r and nun in phage HK022 50
Thesis Ra tionale 52
Chapter 2: Functional Importance of Regions in E.coli Elongation Factor NusA that
Interact with RNA polymerase, the Bacteriophage h N protein, and RNA 53
Abstract
Introduction
Results
Discussion
Expcrimental Procedures
Chapter 3: The a Subunit of Exoli RNA pdymerase Activates
RNA-Binding by NusA
Abstract
Introduction
Rcsults
Discussion
Es perimental Procedures
Chapter 4: Interaction Between the E-colr' NusA Elongation Factor and
the h m r t site RNA
Abstract
Introduction
Rcsults
Discussion
Espcrimental Procedures
Chapter 5: Thesis summary and Future Directions
References
Acknowledgements
I have learned an immeasurable arnount about science from my supervisor, Jack
Greenblatt. 1 started in the lab with very little expenence. Through his signature style of
supervision. 1 have grown into a scientist who feels cornfortable defending her results and
arguing points with peers.
1 would like to thank the many people who have k e n in the GreenbIatt lab who
have filied the years with laughter, support, advice and good mernories.
As fellow collaborators on the prokaryotic project. Joyce Li and Jeremy Mogridge
provided vaiuable reagents and invduable advice.
My committee members. Brenda Andrews, Rick Collins and Barbara Funnell,
have been absolutely wonderful. 1 couid not have asked for more caring and comrnitted
individuals to help guide me through graduate school.
This degree has taken a long time to complete. It would have k e n easy to get lost
along the \vay. but 1 have k e n fortunate to have met many arnazing people. This has
included people both in the Department. as well as ones who are out working in the wild
worId. who kept me grounded with their friendship.
iMy Family has k e n there al1 the way. Thanks for believing in me.
Last. but not least. 1 would like to thank Raja Bhattacharyya for unwavering love
and support.
vii
List of Figures
Chapter 1
The Structural Features of the Elongation Complex
The Regulatory eiements of the h genome
The h. 11rlrR site
The Comple te N-modi fied Antitermination Complex
Chapter 2
Circular dichroism analysis of NusA deletion constructs 58
NMR analysis of fragments of NusA 60
Two RNA poiymerase-binding regions in NusA 62
Rcgions of NusA important for enhancement of termination irl vitro 64
N binds a carboxy-terminal region o f NusA 66
Region3 of' NusA necessary for enhancement of antitermination by N 68
The effects of NusA deletions of the formation of N-NusA-riicr site complexes 71
The effects of NusA deletions on the formation of cornpletc complexes contüining
N. the NusA factors and RNA polymerase 73
Summary of the Results of the Deletion Andysis of NusA 77
Chapter 3
Binding of NusA to RNA in the presence of the RNA polymerase a subunit 89
a-CTD stimultes RNA-binding by NusA 91
RNA-binding by NusA in the presence of a is sequence-specific and
sensitive to a mutation in the S 1 domain of NusA
Interaction of oc with the carboxy-terminal region of NusA
... Vl l l
A carboxy-terrninally truncated NusA binds specifically to rzrrt site RNA 97
The À N protein binds directly to the carboxy-terminal 193 amino acids of NusA 99
Modcl for N USA function in elongation, termination and antitermination 101
Chapter 4
Substitution of nucleotides 1 to 3 in box4 does not affect the ability of NusA ( 1-416)
to interact with rzrrf site RNA, whereas substitution of nucleotides 3 to 6 or
nuclsotides 7 to 9 substantially reduces the interaction 114
Substit~ition of nucleotides 10 to 12 of ho.rA and 13 to 17 of the interbox both
ssverely affect the ability of NusA ( 1-4 16) to interact with nrir site RNA 115
Substitution of nucleotide 23 in the loop of hoxB reduces the ability of NusA ( 1-316)
to interact with n r r r site RNA whereas substitution of nucleotide 31 has
l i t t le effect 117
Substit~ition of nucleotide 27 in the loop of boxB severely reduces the ability of
NiisA ( 1-4 16) to interact with rzrtt site RNA whereas substitution of nucleotides
25 and 26 h a s little effect 118
Mutations in the S 1 homology region of NusA have differential effects on the formation
o t' N-NusA-nitr site complexes 121
Mutations in the S 1 homology region of NusA have differential effects on the formation
of ri complete complex containing RNA polymerase. N. NusA. NusB.
NrisG and S 10
List of Tables
Chapter 4
Comparison of the abilities of NusA ( 1416). in the absence of N. and wild type NusA.
in tIic presence of S. to bind to wild type and mutant nrit sites 119
Introduction
In this thesis, 1 describe experiments performed to elucidate the role of the
E.~clzer-icltilr coli NusA protein in the control of transcription during elongation and
termination as well as during transcriptional antiterrnination medirtted by the bacteriophage h
X protei n. Therefore, 1 kgin with a detailed description of transcription by E-coli RNA
pol y merasc.
Transcrintian in EcoIi
Transcription in Esclwrichia coli is carried out by a single multi-subunit RNA
po1yrncr;isc. This process cün be separated into three distinct stages: initiation. elongation
and tcrmination. Initiation is characterized by RNA polymerase binding to promoter DNA.
conversion ot'an unstably bound RNA polymerase to a stably bound one. and the formation
of the tirsr 1'èw phosphodiester bonds of the nascent transcript. Elongation is the stage whcre
nucleotides are added to the growing chain of nascent RNA at a rate dictated by the template
DNA scqircnce and the factors associated with RNA polymerase during this strige.
Tcrminrition occurs whcn ri signal in the temphte DNA is relayed to RNA polynierase and
the entirc cornplex of RNA polymerase, template DNA. nascent RNA. and any associated
tictors. dissociates. Antitermination prevents the recognition of termination signals by RNA
pol ymerasc. resulting in read-through of terminators. Below, 1 discuss additional control
points that csist at al1 three stages.
initiation
Tlic enzyme responsible for stable transcript elongation is the core R N A polymerase.
a heteronieric protein which consists of one copy each of P ( 15 1 kD) and P' ( l a5 kD)
subunits and two copies of the a (36 kD) subunit. Core RNA polymerase crin associate with
tcmplritc DNA non-specifically. In contrast. specific initiation of transcription is carried out
by thc holocnzyme form of RNA poiymerase (Burgess er al.. 1969). This tom1 consists of
the corc enzyme associated with one of several a subunits. which can specitlcally recognize
proniotcr DNA sequences.
The t?rst step of initiation involves RNA polymerase holoenzyme binding to promoter
DNA. Thc optimal promoter recognized by the major o Factor. known as O"'. has two
clenients: t hc - 10 and -35 elements. and these are separated by a spacer of about 1 7 base pairs
(Haw1t.y and McClure. 1983: Harley and Reynolds, 1987: Lisser and Margalit. 1993). The -
IO clt.nicnt is located 10 brise pairs upstream of the transcriptional start site. The consensus
q u c n c c for this element is TATAAT. Similarly. the -35 element is generdly found 35 base
pairs tipstrcrim of the transcriptional start site and has the consensus sequencc TTGACA.
Thcsc tn.0 clcments makc up the core promoter and act as binding sites for t h c o"' subunit of
the holocnzyinr (Losick and Pero. 198 1 ). Early experïments showed thar a"' cross-links to
thc --3 position of the lacUV5 promoter (Simpson, 1979). and mutations in rpoB. the gene
cncoding 0'". alter the sequence-specificity of the holoenzyme (Cardella er oi.. 1989; Siegele
er cil.. 1989). The interaction of the rpoD mutants with altered promoter sequcnces suggested
ii rclntionship between two regions of a"' that are consewed among a factors and either the -
IO or -35 consensus sequences. Although intact 0" subunit does not bind DNA (Burgess et
tr i . . 1969: Zillig et al.. 1970). an amino-terminally truncated forrn of oÏo can bind DNA on its
own (i.c. in t h e absence of the RNA polymerase core enzyme). suggesting thrit ~ h e amino-
terminal region of ai0 acts as an autoinhibitory domain to prevent DNA binding (Dombroski
ai.. 1992: Dombroski et cd.. 1993). Thus. interactions within the RNA poly rnèrase
Iioloenzymc prevent autoinhibition of DNA binding by the amino-terminal region of a'' and.
in this iviiy. specific promoter recognition by the holoenzyme becomes possible.
A third promoter element upstream of the -35 element was recently discovered in the
promoter for an rm gene. This sequence. the UP element. is found in the pronwters of a
wbsct of gcnes. It is located roughly 55 base pairs upstream of the transcriptional start site in
t hèsc pronioters (Rao et d.. 1994). Although there is no strict consensus seqiience for thé UP
clcmcnt. i t is characterized by AT-richness. UP elements can affect the stren~th of a
promotcr by ris much as thirty fold. It is postulated that the increase in promotcr strength is
providcd by a direct interaction between the UP element and the a subunit of RNA
pol y nie~isc I Ross et c i l . . 1993; Murakami cr al., 1996).
The initial RNA polymerase-promoter DNA complex is unstable and is called the
closcd coniplex. This closed complex has at least two conformational states. The initial
closed cornplex has a footprint on the DNA from -55 to -5 and can only be stiidièd at low
tcmpcraturc (Hofer et ai.. 1985: Cowing et cri.. 1989; Schickor et cil.. 1990: Mecsas et cri..
199 1 ). As the temperature is increased. this initial complex becomes an isomerized closed
complcs thrit has a footprint from -55 to +?O. thus encompassing the start site of transcription
at position + 1 (Cowing et cri.. 1989; Schickor et al.. 1990; -Mecsas er al.. 199 1 1. Further
conforniarional changes within this closed complex lead to isomerization into ri more stable
open conipIcs in which part of the region of DNA to which the polymerase is bound is
mclted. thcrcby exposing the nucleotides to be used as the template for the transcription
rcxtion. The entire process of initiation can be summarizcd in an equation that refiects the
rates rit \$.hich thcse complexes fonn:
where R=RNA polymerase. P=the promoter. RP,=the closed complex. RP,=the open
complex. K,,=the equilibriuin constant that describes the binding of RNA polymerase to the
promoter DNA and kpthe rate constant for the isomerization between the closed and open
complexes.
Once the open complex has formed. a process cailed abortive initiation can occur.
This process results in the cyclicd creation of short transcripts from 2 to 9 nuclcotides in
lcngth \vithout the loss of the specific RNA polymerase-promoter contacts that chwacterize
the open coniplex (Johnston and McClure. 1976; Carpousis and Gralla. 1980). Initiation is
cornpiete n~hen RNA polymerase transcribes beyond the first 8 to 10 nucleotidcs of initial
scquencc and escapes from the promoter.
E d y studies in regulation of transcription focused on control rit the Icvcl of initiation.
Acti~~atorï and repressors use different mechanisms to affect steps outlined in rhc equation
above. For instance, the 1- repressor protein. CI. activates the h promoter PR,, by increasing
the rate constant of the isomerization step. The CAMP-binding protein. CRP. activates the
E c ' o l i ICIC' promoter by increasing the binding constant for RNA polymerase at this promoter
(,Malan L'I C I / . . 1984: Li et (11.. 1994). Interestingly. another h protein. cII. activritcs the h
pronmter P,, by increasing both the K, and the k, (Shih and Gussin. 1984). Repressors can
\i-ork b ~ . siniply binding to the promoter and sterically blocking access of RNA polymerase.
In addition. repressors can affect the kinetics of initiation. For instance. CI inhihits the
formation of an open complex at the hP, promoter, while the lac repressor modilies the open
complcs siich that it cannot escape the promoter (Hawley et cil.. 1985: Lee and Goldfarb.
1991).
Thcse effects on initiation can result from a direct interaction of the activator or
rcpi-cssor iiith one of the subunits of RNA polymerase. The target of CI at kP,,, is a'"(~i L'I
( 1 1 . . 1994 1. \\.hile the carbosy-terminal domain (CTD) of cf: is the target of replatory proteins
such ris the CAP protein and the FIS protein (Li et al.. 1994: Bokal et cd. . 1997). FIS is a
DNA-binding protein that most notably increases the amount of transcription at the already
strong rr11B P 1 promoter (Ross et al., 1990). This promoter is regulated by an LtP element-a-
CTD interaction and FIS may facilitate a-CTD binding to the UP element through interaction
with u-CTD (Bokal et al.. 1997).
Anotlier level of controt of initiation of transcription is represented by O factor-
binding. There are two families of E.coli a factors: the &'"' hmily is required for nitrogen-
regulatsd gcne expresssion and the O" farnily of prîmary factors is required for vegetative
growth und rcsponse to environmentai conditions and physiological States (Gross et al..
1992). Association of these factors with the core enzyme directs the holoenzynie to
conscmcd sequences in promoter DNA specifically recognized by individual o factors.
TIiese o tactors. in turn. are regulated by the action of anti-O factors that bind and sequester
specific O factors until they are needed. Anti-6 factors can atso inhibit eariy steps in
clonsation. FlgM is a flagellar anti-0 factor h m Salmorwllrr rydtiniririunr. In addition to
binding to its cognate o Factor. &'. it c m bind a &s-holoenzyme molecule and dissociate it
(Chads1t.y el ul.. 1998). Another strategy used by anti-o factors is to prevent association of
thc O suhunit with DNA once it is part of the holoenzyme. The bacteriophagc T, protein.
AsiA. binds the 070-holoenzyme once is has already fomed but occludes the -35 recognition
site ( Sevcrinova et al.. 19%).
Elongation
Thc transition from a promoter-bound initiation complex to an elonpation compiex
criprtblc of rapid RNA chain elongation is ~ h a ~ c t e r i z e d by the loss of the a subunit. loss of
promotcr-specific contacts. ü change in the size of the RNA polymerase footprint and an
incrras in ih r stability of the complex to a variety ofconditions. including hizh salt
concentrations (Hansen and McClure, 1980: Carpousis and GraIIa. 1985: Strancy and
Crothers. 1985; Levin et trl.. 1987: Arndt and Chamkrlin. 1990). This incrcasc in stability
has facil i tated the purification of the elongation complex. resulting in the elucidation of some
of its srrucriira1 features.
As dcscribed beloiv. three models have k e n proposed to expiain ho~v RNA
polymcmsc transcribes DNA: the rnonotonic. inchworrn and backsliding models. In the
rnonotonic riiodel the elongation complex was proposed to be relatively inflexible and to
move dou-nstream on the template in single steps. Therefore. addition of a nucleotide to the
cro~ving R S A chain results in the forward movement of t h e complex by one nucleotide. the C
maintcnancc of a 17 nucleotide bubble of unpaired DNA by the actions of a do~vnstream
"iinu.indasc" and an upstrearn "rewindase". and the maintenance of the 8-1 2 nucleotide
DNA-RNA hybrid (Kumar and Krakow. 1975; Gamper and Hearst, 1982: Hanna and
Mcarcs. 198-7: Kainz and Roberts, 1992). This model suggested that the charmeristics of the
cori1p1c.u. wch as fooiprint size and stability to salt and dissociation. would renuin constant.
Ho\vc\w. tiirther studies of many complexes isolated from different positions on a given
tcmp1:itc sccmed to contradict the monotonic model. since some complexes had apparently
diftrcnt sizcd DNAse 1 footprints. different mobiiities in a polyacrylamidc sel. different
\tabilitics to dissociation. and different potentials to resume transcription after being halted
( Krumnxl and Chamkrlin. 1992b; Krummel and Chambcrlin. l992a). Thcsc rcsults
suggcstcd ri rnore dynamic complex, one that is sensitive to tcrnplrite position. Thus. based
on thcsc and other observations, Chamberlin proposed the inchworm model. which
postulated that elongating RNA polymerase is flexible and could cornpress and csxpand.
rillowing nlovement of its upstream end independent of its downstreani end (Ciiamkrlin.
1992 1.
The discontinuity of elongation complex movement associated with inchwoming
was postulatcd as a result of nascent RNA chain extension that seemed to remitin continuous
even at tcrnplate positions where the front end of RWA polymcrase no longer rnoved fonvard.
The addition of nucleotides CO the nascent RNA chain seemingfy caused thc active center of
RNA polynictase to move tonvard on the DNA template. without the corresponding fonvard
movcmcnt of the front end of the elongation complex, creatinp interna1 strsiin that was
seemingly rcIieved by a fonvard "leap" of the complex at certain positions on tlie templatc
(Nudicr c r (11. . 1994). Central to this model was the idea that the template con\.eyed
information to create this inchworrning effect. Comprehesive studies at every base position
on a spcciiic template showed that RNA polymerase advmced in a monontonic mode at most
D S A scqucnccs. but that, at certain DNA sequences, most notably at pause and termination
sites. RNA potymerase switched to an inchwonning mode (Nudler et cri.. 1994: Nudler cJr cri..
1995: ii7;ing et trl.. 1995).
Thc bricksliding model reinterpreted the above data to explain the mo\.c.ment of RNA
polymcrasc dong the template DNA in a different way. In this modcl. RNA polyrnerase is
cnvisioncd to oscillate bet\veen inactive. arrested complexes that have slid back~vards on the
DNA tcmpl~ite and active. clongation-competent complexes ( Komissarova and Kashlev.
l997b: Koniissarova and Kashlev. 1997a). Weakening of the DNA-RNA hybrid increased
backsliding. while strentthening the hybrid decreased the incidence of backsliding.
s u g p t i n g that the signal to enter the oscillation cycle is based on the relativc strength of the
DNA-RNA hybrid (Nudler et al., 1997). Two lines of evidence support this model. First,
the DNase 1 footprint of a halted eIongation complex moved upstream. with the front and
back ends of' the complex backing up in concert with each other: second. the critalytic center
of RNA polymerase also moved backwards. so that the 3'-terminus of the RNA was extruded
tiom thc iicti~pe site (Komissarova and Kashtev, 1997a: Komissarova and Kashlev. 1997b:
Nudler cl c l / . . 1997).
Thc apparent contraction and relaxation of RNA polymerase postulüted by the
inchworriiing mode1 was most likely due to misinterpreting the footprinting of only the front
end of' the complex at difkrent points in its oscillation cycle. As well. it was originally
assumcd that the active site of RNA polyrnerase would ren~ain attached to the 3' end of the
nascent RNA. leading to the Calse assumption that the position of the catalytic centre was
dcfined by the position of the 3' end of the RNA. However. this was formally disproved
using an RXA polymerase in which the active site Mg2+ was replaced by ~ e ' * . causing
localizcd clciivage of nucleic acids (Zaychikov et al., 1996). In the arrested coriiplexes. Fe"-
niediritcd clciivage of the tninscript upstream of its 3' end strongly suggested that the active
site had indced moved upstream and that the cleaved RNA represented extrudcd RNA
(Niidlcr cr ( 1 1 . . 1997). The backsliding rnodel is currently the accepted model of'elongation
and prmeidcs the frrimework for models of termination and antiterrnination (scs below).
Thc clongation cornplex is a stable structure at most positions along the DNA
ternplatc. TIic nature of thc rnolecular interactions that provide stability and proccssivity for
the elonption complex are of interest because loss of these interactions has implications Ior
the replation of transcription. The monotonic model of elongation suggested that the 8- 13
base pair DNA-RNA hybrid confers most of the stability necessary for the synthesis of long
transcripts. whereas DNA-protein and RNA-protein interactions contribute only ii srnall
measurs to the stability (Yager and von Hippel. 1987; Yager and von Hippel. 199 1 ).
Dismption of the hybrid would lead to destabilization of the complex and events such as
arrest. pausing and termination. However. subsequent studies suggested that the DNA-RNA
hybrid niay not be the only stabilizing factor for the cornples. For instance. one audy
reportcd that RNA polyrnerase c m bind the 3' end of RNA in the absence of a DNA ~emplate
providing c~.idencc for an RNA-binding site within RNA polymerase that contributes enough
stribility to allow for functions that normally occur in an elongation complex (Altmann er c d . .
1994). The RNA in the complex can be cleaved and extended, suggesting that the cornplex is
rit lcast somcwhat stable. To explain these observations. the inchworrn modcl had postulated
that sepxitc RNA- and DNA-binding sites hold the complex together to elirninaie the need
tor a long hybrid (Chamberlin. 1992).
Figure 1. The Structural Features of the Elongation Complex. A schematic of the features of the elongation complex important for stability of the complex. DBS is the DNA-binding site, RBS is the RNA-binding site. HBS is the hybrid-binding site and Mg+ represents the active site.
Tlic present view oC the structural aspects of the elongation complex is shown in Fig.
1. There are three main features in RNA polymerase that contribute to stability of the
compIcs: the hy brid-binding site, the double-stranded DNA-binding site and the single-
stranded RYA-binding site (reviewed in Nudler, 1999). A DNA-RNA hybrid 8-9 nucleotides
in length h~is been confirmsd by cross-linking RNA to DNA and by protection tiom cleavrige
by diffctrent RNases (Nudler et ai.. 1997; Komissarova and Kashiev. 1998)- The hybrid-
binding site in RNA polymerase maintains the RNA and DNA in the elongation complex
linder l o ~ v sril t conditions (Nudler er cd. . 1996). The double-stranded DNA-binding site
interacts tvith 10 base pairs of duplex DNA just downstream of the active site of' RNA
p o l ÿ n ~ c ~ i s c and is responsible for the stability of the cornplex in high conccntrririons of salt.
Finallq.. thc single-stranded RNA-binding site adds more stability against high sait
conccntrritions. The obsenvation that RNA that is upstream of the DNA-RNA hybrid is
import~int !or stability of the elongation complex provides the framework for one of the
current niodcls of transcription termination (see below).
Thc interactions that hold the elongation complex together presumab
maintain the processivity of transcription. Once it has initiated transcription
pol>*n-icrasc may maintain al1 these contacts until a terminator is reached. Howcver. there are
sites rilong the template thrit can cause RNA poIymerase to pause on the DKA !or v q i n g
amounts of time or to m e s t transcription completely. Arrcst sites are defined as sites where
R N A p o l ~ ~ m r a s e stops trrinscription irreversibly in iirro. The elongation coniplex remains
intact rit such sites but is unable to recommence elongation. even when supplied with al1 the
requircd nucleotides. because the transcript is out of register with the active site (Arndt and
Chamberlin. 1990; Kornissarova and Kashlev, 1997b). This situation has not been observed
iri i-ira. Ittading to the hypothesis that additional factors may reactivate arrestcd complexes N i
\.i\.o. Thc clèavage factors GreA and GreB are candidates to fulfill this function.
GrcA was identified in a screen for suppressors of a temperature-sensitive mutation in
the p suhunit of RNA polyrnerase (Spxkowski and Das. 1990) and was subsequently shown
to induce cleavage of the 3' end of the nascent RNA in wested elongation complexes
(Bonikhov et ~rl.. 1992). This finding was rapidly followed by the discovery of a second
clea\*a_cc factor. GreB, and the characterization of the propenies of both protcins (Bomkhov
er cil.. 1993 1. Although RNA polyrnerase itself possesses intrinsic cleavage activity (Orlova
et cil.. t 995). GreA induces removal by RWA polyrnerase of 2 to 3 nucleotides from the 3'
cnd of thc nascent transcript. GreB induces cleavage, also by RYA polymerrise. of up to 9
nuclcotides tiom the 3' end (Bonikhov er al.. 1993). According to the backsliding mode1 of
clongrition. ;in arrested complex is one in which the 3' end of the nascent RNA is o u t of
rcgistcr ~ v i t h the active site (Komissarova and Kashlev. 1997b). and the Gre L'rictors probably
inducc clcaviige of the extruded RNA by the active site (Nudler et al.. 1997).
R S A polymerase pauses for various lengths of timc at pause sites. but then can
rssumc transcriptional elongation after escape from the pause. it h3s been suggcsted that
pac~sing is ;i way to maintain the tight coupling between trrinscription and translation that
occurs in bxtcria. One clear example in which pausing has this function is a~tcnuéition.
wherc rcgu1ation of a single terminator occurs in response to a metabolic signal (Landick and
Turnbough. 1992; Landick et ai., 1996). Attenuation often regulates the transcription of
amino ricici biosynthetic operons and, in these cases, there is a rho-independent terminator
locatcd bétu-cen the promoter and the first gene of the operon. A pause site is located
upstream of the terminator and. since translation initiates upstream of the pause. tight
coupling or transcription and translation is ensured. Once transcription resun~cs. alternate
RNA sccondary structures have the potential to form. one Ieading to termination and the
other to tcrminator read-through. and this decision is based on the availability of the relevant
amino acid. If the concentration of the amino acid is Iow. the ribosome stalls neithin a critical
leader peptide. allowing the antiterminator hairpin to form. If the concentration of the amino
x i d is high. transiation continues through to the stop codon and formation of the teminator
hairpin is niostly fhvoured.
At one kind of typical pause site. a hairpin structure forms in the nascent RNA whose
3' end is scparated from the 3' end of the RNA in the paused elongation complex by 10 or 1 1
nuclcotidcs. Whether RNA polymerase will recognize a pause site and how long it will
remain at thc site is affected by the nascent RNA hairpin. the RNA located hctiveen thc
hairpin anci the 3' end of the tnrnscript. the identity of the incoming nucleotide and the DNA
downstrcarn of the active site (Chan and Landick. 1993; Levin and Chamberlin. 1987; Chan
and Landick. 1994).
.As RNA polymerasc approaches a pause site. it undergoes an alteration in
conforn~ation and begins to backslide (Wang et ni.. 1995). As transcription elongation slows
duc to ilic oscillation cycle. the pause hairpin has time to form. Furthermore. the pause
hairpin is thought to block tùrther backsliding by allowing the formation of a stable. paused
complcs. Rccent observations suggest that there is a direct interaction betwcen the RNA
hairpin and R N A polyrnerrisc and that this interaction stabilizes the pause conformation
(Chan CI (11 . . 1997; Wang cJr tri., 1997; Artsirnovitch and Landick. 1995).
In addition to signals derived from the DNA and RNA. the length of time that RWA
polymcrrisc rcmains at a pause site can be influenced by cstcrnal factors. Nusi\ associates
with RSA polymense after escape from the promoter and loss of a'' (Greenblatt and Li.
198 1 a). As part of the elongation cornplex. NusA decreases the elongation rite of RNA
polymerasc. This phenomenon occurs in two ways: NusA increüses the length of time that
RNA polynier;ise remains at certain pause sites (Kassavetis and Chamberlin. 198 1: Kingston
and Chamberlin. 198 1 : Farnharn er ni.. 1982: Lau et al.. 1983: Landick and Yanofsky. 1984:
Landick and Yanofsky. 1987: Levin and Chamberlin. 1987: Faus er cd.. 1988) and it
increases the substrate dissociation constant (Schmidt and Chamberlin. 1984). I t is not clcar
how NusA inlluences the response of RNA polyrnerüse to pause signals. but it is postulated
that KLISA interacts with the pause hairpin to stabilize it (Landick and Yanofsk~ 1987: Faus
et crl.. 19SY ). This mode1 is based on the observation that the RNase T, digestion pattern of
an isolatcd pause hairpin is altered by the presence of the E.'.co(i NusA protein i Lrindick and
YanofSk!f. i 987). These effects on the RNA poiymerase elongation rate can influence gene
rcgulrition and may be neccssary to couple transcription and translation (Rutcshouser and
Richardson. 1989: Zheng and Friedman. 19%).
NusG is another elongation factor that interacts directly with RNA polymerase (Li et
(11.. 1992 1. However. its et'fect on transcription elongation is opposite to that 01' NusA. NusG
incrcascs thc elongation rate of RNA polymerase in i?i\-o and N i i'itro (BuroiPa cl trl. . 1995 1.
Tcrriiination occurs when a signal in the DNA template causes RN.4 polymerase to
stop thc addition of nucleoside triphosphates and to dissociate fiom the DNA. In E-coli.
there are two types of termination signals. ones that do not require accessory factors in order
to functioii and others that require the termination factor 12-10. The former type of terminator
is called an intrinsic. simple. or rho-independent terminator. whereas the latter is called a rho-
dependcnt tcrminator.
Rho-independent termination
lntrinsic terminators have some features in common with pause sites. They typically
contain a GC-rich stem loop structure located 7 to 8 nucleotides upstream of the termination
site. Houw-er. termination sites also contain at least 3 uridine residues downstrcam of the
tcrniinator hiiirpin and this fcature is not present in pause sites.
An carly mode1 for termination at intrinsic termination sites postulated that the
Formation of the terrninator hairpin destabilized the DNA-RNA hybrid and that this
destabilizrition. as well as destabilizing effect of the rU-dA base pairs in thc DNA-RNA
hybrid. causcd dissociation of the nascent transcript. Thus. in the absence of the uridine
stretch. ihc pause hairpin n.ould be sufficient to cause a break in elongation at a site Iocated
tùrthcr don-iistream. but u-ould not be sufficient to cause dissociation of the cornplex.
The initial view was that elongation and termination were competitivc proccsses. and
that at evcqr position on the template there was a thermodynamic bamer to dissociation of
the DNA-RXA hybrid (von Hippel and Yager, 1991: Yager and von Hippel. 199 1 ). The
cnergy rccluired to maintain elongation was derived from the sum of the energ!, lost in the
format ion of the transcription bubble and the energy gained in the formation of the DNA-
RNA hybrid. This energy barrier was proposed to be higher at termination sites than at
clongation sites. favouring dissociation of the complex only at a terminator.
Honwer. recent experirnents have suggested an alternative model. Instcrid of only
destabilizing the DNA-RNA hybrid, it has been proposed that the terminator hairpin also
disrupts the protein-RNA interaction within the RNA-binding site (Yarnell and Roberts.
1999). Thus. the hairpin would effectively extract the FWA from the transcription complcx.
Protein-RSA cross-linking experirnents are consistent with this observation: formation of ri
terminator hairpin aiters the pattern of cross-linking betwcen RNA polymerasc and the RNA
in thc RKA-binding site (Gusarov and Nudler, 1999). Hairpin formation would also lead to
the unwinding of the DNA-RNA hybrid and transcription bubbie collapse. Thc second
terminntion determinant. the poly-uridine stretch. stalls RNA polymerase to allow tirne for
thc formation of the hairpin. This pause may occur when the weak ru-dA basc pairs in the
DNA-RSA hybrid allow RNA polymerase to slide backwards and disengage its catalytic
ccntrc froni the 3' end of the RNA. If the hairpin is deleted. no termination is observed but
the priusc still occurs (Nudler er cd.. 1995; Yarnell and Roberts. 1999); likewisc, if the rate of
clongation is very low. or thc complex is nucleotide-starvcd and static. termination occurs in
thc ahscncc of the uridine-rich region (Yarnell and Roberts. 1999). Thus. the new model
postulatcs thrit the elongrition complex pauses because of the poly-uridine tract in the nascent
RNA, Tlic RNA hairpin t hcn forms and the complex dissociates because of' t hc i nstabiiity
crcritcd by the loss of RNA-protein contacts in the RNA-binding site and the cinwinding of
thé DSA-RXA hybrid (Gusarov and Sudler. 1999; Yarnell and Roberts. 1999).
AIthough no additional factors are required for termination at intrinsic tcrminators.
NusA can increase the efficiency of termination at many of these sites (Greenblatt er cd. .
198 1 : Farnham et al., 1982; Grayhack et al., 1985; Schmidt and Chamberlin. 1987). The
mechaniml by which NusA increases the termination efficiency is not well understood. In
this thesis. I show that the ability of NusA to interact with RNA polymerase and with RNA is
likely to bc important for this process.
R ho-dependent termination
Rho \vas identifieci as a factor that causes transcription termination and the release of
the RNA in a transcription reaction in vitro (Roberts. 1969). Rho is a hexzimeric protein that
binds RKA (Finger and Richardson. 1982). It possesses DNA-RNA helicasc rictivity and
RNA-dcpcndcnt ATPase activity (Lowery-Goldhammer and Richardson. 1974: Brcnnan et
(il.. 1957 ). These properties suggested that rho binds the nascent RNA upstrcrini of the
olongating RNA polymerase. translocates alont the RNA toward the RNA polymerase
powercd hy its ATPase activity. and dismpts the elongation complex by using its helicase
activity (Plritt and Richardson, 1992).
Scq~~cnces that are important for the generation of rho terrninated transcripts have
bcen idcntilied by testing t'or the effects on rho-dependent termination of either deIeting
portions of the upstream DNA or using DNA oligonucIeotides that Iiybridizc \vith various
portions of the upstream RNA. In this way. two rut (rho-urilizrition) sites tvw-c identified
upstrca~ii of the rho-dependent h terminator tR 1 (Chen et cri.. 1986; Chen and Richardson.
1987). Tficse sites. calied t-[UA and nttB. are characterized by a relatively high cytosine
contcnt and ~ e r y little secondary structure (Chen et al.. 1986; Chen and Richardson. 1987).
In fact. cs:imination of al1 defined rho-dependent temination sites, as wcll as of cryptic rho-
dependcnt termination sites within the his operon, led to the conclusion that rho-dependent
terminritors consist of cytosine-rich and guanosine-poor regions with iittle s e c o n d q
structure upstream of defined rho-dependent 3' endpoints (~Morgan el cil.. 1985: Alifano ur cil..
199 1 1. in contrast. no defined sequenccs have been identitled for the trariscript 3' endpoints
(Richardson and Richardson. I996), which rire often sites where RNA polymcrase pauses
(Lau et tri.. 1882: Lau er { i l . . 1983). This finding suggests that slowing RNA poIymerase
al io~vs rho 1 0 reach the elongrition complex downstream of the site whcre rho initially
interacts tvith the nascent transcript. This view is supported by the observation that there is
an invcrsc rclritionship between the elongation rate of RNA polyrnerase and the efficiency of
sho-depcndcnt termination (Jin et al., 1992). However. there is no direct relationship
bctu-ccn thc strength of a pause site and the ability of rho to cause termination. suggesting
thüt thc conformation of the paused complex is the target of rho (Richardson and Richardson.
1996). This view is supported by the finding that RNA polyrnerase probably dides
backn.ards at pause and termination sites (Nudler et al., 1995: Wang er cil.. 1995 1.
Thc cryptic rho-dependent terminators in the /ris operon were identitied through
niutrit ions thrit caused polarity and subsequent rho-dependent termination of tmnscription
( Alifrino ïJr (11. . 199 1 j. Polarity is observed when prematurc termination of translation of an
crtrly gcnc in an operon results in the reduction of transcription of distal gencs in the opcron.
Thc obscn-iition that riio mutants suppress polarity in the ,sa1 operon (Das el al.. 1976)
suggcstcd that rho binds to untranslated RNA (Adhya er cil.. 1974).
Dcplction of NusG. originally identified as a bacterial protein important for h N-
mediatcd rintiterminarion (Li et d., 1992)(see below), reduced rho-dependent tcrmination at
nlost tcrn~inators that were tested (Sullivan and Gottesman. 1992). suggesting that NusG is
important for rho function iri vivo. Subsequent in vitro experirnents confirmed that, at sorne
tcrrninators. under conditions where rho does not function well on its own. NusG enhmces
the terminrition efficiency (Burns and Richardson. 1995) by binding weakiy to rho and RNA
pol y merase ( iMason and Greenblatt. 199 1 ; Li et al., 1992: Li er al.. 1993). These interactions
may aid rho factor in locating the transcription comptex (Li rr al.. 1993). Altsrnatively. t h e
interaction between rho and NusG rnay stabilize the binding of both proteins to the
èlongation cornplex. In support of this model, NusG does not associate wcll lvith the
clongation cornplex unless rho is present and bound to the nascent RNA. and NusG slows the
off-rate OS rho tiom staIled elongation complexes (Nehrkc and Platt. 1993). Ftirtherrnore.
XusG stiifis the positions of the endpoints of the terrninated transcripts in the 5' direction (Li
ct (il.. 1993: iiehrke et al.. 1993). Perhaps. NusG enables rho to bind more quickly to its high
aftinity binding sites in the nascent RNA as they become rivailable during transcription.
Remdation of Elong&m h~ the Racterio~haee N rotei in
Antitcrmination is an important form of regulation during the elongaiion phase of
transcription. As a resul t of this process. RNA polymerase transcribes through tcrminators
on thc DNX iemplate. Thc best studied systems of antitermination are found in
bactcriophiigc A. When h infects an Ecoli cell. transcription of the phage gcnome is carried
out by the host RNA polymerase. Initiation of transcription occurs at PR and P, . the
promotci-s of' the rightward and leftwxd phage early operons. rcspectively (sce Fig. 2 ) . The
initial transcripts are short because the rho-dependent terminators. tR, and tL,. are located
downstrc:ini of the first genes in the two operons. However. the N gene transcript is present
in the initially transcribed PL RNA and it is the presence of the N protein that is key to the
transformation of RNA polymerase into a termination-resistant form. The action of N is
requircd for the expression of al1 downstream genes in both operons and. in the pxticular
case OS the rightward operon. for the positive regulator of the late genes, the Q protein.
Figure 2. Regulatory elements of the h genome
Proccssive N-mediated mtitermination. or antiterrnination that persists through many
tcrminators and for many kilobases of sequence. requires the presence of a cis-acting
clement. ciilled the r z r r t site (N-rrriiisation). that is located in both of the early operons.
Proccssivc antitermination also requires several host factors. called Nus (N-rrtilization
substancc 1. that were identitïed in screens for host mutations that prevented thc growth of
niId tjrpc Y-dependent phage. but not the growth of N-indcpendent mutant phage. N-
rnediatcd mtitcrrnination prevents termination by RNA polymerase at both rho-dependent
and rho-indcpendent teminators.
The nut site
The temperate bacteriophages A. PI2 and 92 1 have very similar genonie
orgmizations. However. ivhereas the regulatory proteins of these phages havc analogous
tunctions. the sequences have diverged so that a regulatory protein frorn one phage often
cannot fiinction in another. Because the phage DNA sequcnces have limited homology. it
has bcen possible to create hybrid phages to anütyze relationships between particular
regulatory proteins and thcir sites of action. The requirement for a cis-acting scquence for
rcgulation by N was first postulated when i t was shown that h N could not fiinction in othcr
larnbdoid phages (Friedman et al.. 1973; Friedman and Ponce-Campos. 1975 1. -Mutations in
non-coding sèquences that prevented the functioning of N then led to the disco\.cry of mctL
and nurR (Rosenberg et crl.. 1978; Salstrom and Szybalski. 1978). The rzirt sites are locüted
betuwn t h c promoters and the first terminators of both early operons. Subscq~ient
conip:irison of the nrrt sequences of the related coIiphages A. 92 1 and P22. as \\.cl1 as
niutational studies. led to the identification of two separate elements of the urrr site. the 12
nucleotidc ho.vA element and the 15 nucleotide boxB element which can form a hairpin
structiirc in the nascent RNA (Franklin. l98Sa; Olson er trl.. 1982)(Fig. 3). Thc sequencc
bctwecn /1o.v,4 and hosB is called the Nirerfmr. Since the cloning of nlctR downstreüm of a
hetcrologoiis promoter in the absence of PR did not alter thé ability of the nrrt site to support
mtitcrmination by N. it was concluded that the nrit site itself is suftïcient for antitermination
t dc Cronibrugghe et al.. 1979).
C G l C G C U C U U A C A C A U U C C A G C 7
e- boxA interbox
Figure 3. The )c nutR site
Various experiments have defined rzrrt sequences rcquired for antitsrniination. For
csanipk. pxticuiar mutations in any nucleotide in the loop of the hosB hairpin completely
abolished antitermination (Doelling and Franklin, 1989). Deletion of eight nucleotides of
1~o.\;c-i scqucncc lefi antitermination unaffected in one study. suggesting to thc riuthors that
Iwi.\il is not rcquired for antitermination (Zuber et al., 1987). However mutations in both
lx~i-A and iurc.rh.r nucleotides affect the functioning of the rlrrt site irt vitro (Chapter 2 ) .
iMoreoi.tlr. the loss of hn.d rendered antitermination therrnosensitive irz i i l w i n another study
t Pcltz er trl.. 1985). Thus. / 7 0 . ~ 4 is important for antitermination while hoxB is essential.
Se\-er-al lines of evidencc have suggested that the functional form of thc rllri site is
R N A . Firht. a frameshift mutation that allowed translation through the rrlrt site prevented
anti terminrit ion (Olson et trl.. 1982). Furthermore. overexpression of ho-rA RNA decreased
mtitermination in i t i i . 0 (Friedman et al.. 1990). and high concentrations of RNwc relertsed N
from Y-nlodified transcription complexes irl virro (Horwitz et al., 1987). Morc direct
a.idcncc has been providcd by gel retardation experiments in which N and the Kus factors
bound to and retarded the electrophoretic mobility of the 11rr1 site RN.4 (Chattopadhyay et c d . .
199%: Mogridge et al.. L995; Tan and Frankel, 1995: Mogridge et al., 1998b) and by the
obserxxtion that N and the Nus proteins can protect the r i rrr site from degradation by Wases
iirrr) (Nodwell and Greenblatt, 199 1 ).
ho.\-R is a 15 nucleotide hairpin with a 5 nucleotide loop and a 5 base pair double-
strandcd stem (Chattopadhyay et al., 1995a: Su et al., I997a: Su et cri.. 1997b: Lcgault Cr cr i . .
1998: .Mogridge et al.. 1998a). bo.d appears to be linear at its 5' end. as detcrrnined by
structure probing with single-stranded RNA-specific and double-stranded RNA-specific
ribon~icltlasés. Since nucleotides at both the 3' end of boxA and within the irltcjrix~x spaccr
re~ion arc sensitive to both single-stranded and double-stranded RNA-specific rcagents. i t
\vas suggcstcd that they may alternzte between single-stranded and double-stranded
conformations (Nodwell and Greenblatt. 199 1 ).
Tlic N proteins of phages h and 2 1 cannot functionally complement each other or the
X protein of phage P22 (Dambly and Couturier. 1971). The i3o.d sequencc is conserved
:irnong thc Iambdoid phages. as well as among the seven Ecoli nw operons. suggesting that
/x).I-A contains sites of interaction with host factors (see below). In contrast. thc predicted
structure but not the sequence of hoxB is conserved among ihe lambdoid phages suggestins
that hosB is the recognition site for N. The interbox is not well conserved and its function
hrts not bcen dcscribed. However, gel mobility shift experiments with mutatcd urrt sites have
suggcstcd that these nucleotides are important for the association of host factors with the Y-
rnodi ticd antiterrnination complex (Chapter 4; Mogridge et id.. I998b). Furthcrmore.
deletion of some of the nucleotides in the Niterbox prevented rintitennination itz r-irto
(Doelling and Franklin, 1989).
The Processive Antitermination Cornplex
Many protein-protein and protein-RNA interactions characterize the processive
antitcrminrition cornplex. .Most of these interactions were initially idcnti fied gcr,ètically and
thcn wriiicd by various biochernical assays.
N protein
S proteins from related phages are al1 srnall, basic proteins but their sequences arc
highly divergent (Franklin. 1985b). However. a lysine- and arginine-rich region with high
intcrspccies similarity is essential for N to tùnction in igiiw (Franklin. 1993). Arginine-rich
motifs arc present in several RNA-binding proteins, such as the HIV Rev and Tilt proteins
(Tan and Frankel, 1995). The structures of Rev bound to its cognate site. RRE (Battiste er
(ri.. 19% 1. and Tat bound to its RNA site, Tar (Puglisi et (11.. 1995). showed that different
Arg-r-ich niotifs adopt diffcrent structures that bind differentially to RNA. suggcsting that the
motif a1lou.s for flexibility in binding. Direct evidence that N binds bnsf3 was provided by
domain suapping experiments in which the amino-terminal region of the N protein was
repliiccd hy the homologous region from the $3 1 N protein (Lu insk i et (11.. 1989). This
hybrid X protein was only fiinctional when the boxB sequencc from 021 was prcsent
(Lazinski ( I r trl . . 1989). Subsequently, gel mobility shift cxpcriments demonstrateci a direct
and speci fic interaction betwern N and boxB RNA. The electrophoretic mobility of the wild
type ilrlt site RNA was altered upon the addition of N protsin. whereas mutations in
nucleotidcs 1. 3. or 5 of the 5 nucleotide loop of boxB prevented this alteration
(Chattopadhyay er al.. 199%: Mogridge er al.. 1995). The minimal peptide that could bind
hrd3 neas the amino-terminal 22 amino acids. contiming the carlier suggestion that this
arginine-rich region interacts with the RNA (Chattopadhyay et cd.. 199%: Tan and Frankel.
1995 ).
3 i 'i relatively unstructured until it comes into contact with the RNA (Su et al..
1997a: Van Gilst et al.. 1997: Mogndge et cil., 199th). Furthermore. the structure of N ( 1 -
22) bound to llo.rB site RNA. as so1ved by NMR. indicated that its arginine-rich region
ridopts an a-heiical structure in the bound state (Su et al.. 1997a: Legault er (11.. 1998:
.Mogridgc cr al.. 1998a). The rest of the h N protein remains unstructured whcn N binds
IIosB R N A (Mogridge et al., 1995) presumably until N cornes into contact with other binding
partncrs such as NusA and RNA polymerase (see beIow). The structure of ho.rB is also
altercd ripon N-peptide binding (Su er cil.. 1997b; Legault rr cil.. 1998: Mogridgc C r cri..
199Sa). Insicad of the predicted 5-membered loop. hoxB forms a stable G N R A tetraioop-like
structiirc lvith the guanosine in the first nucleotide of the loop and the adeninc in position five
of thc predicred loop forming a non-Watson-Crick base pair (Su et al.. 1 9 9 7 ~ SLI et al..
I997h: Lc(rau1t L er al.. 1998). The fourth nucleotide of the loop. a guanosinc in r~rrtL. is
loopcd oiii. is not part of the structure recognized by N, and is not necessaS. Ilv the GNRA
tctraloop-like structure to form (Legault er til.. 1998). Since mutation of this niicleotide is
dctrinicntiil to antiterminrition by N in ~ i i w (Doelling and Franklin. 1989) and prcvents the
association of NusA with an N-nlrt site complex in virro (Mogridge et trl.. 1995: Legault et
(11.. 1998). it iJ likely that it is important for interaction with NusA. Interestingly. the base
stacking interactions in the A-form double helical stem oflm-vt? are extended hy the G-A
base pair t hat closes out the loop and loop nucieotides 2 and 3. These interactions are
additionall). stabilized by the indole ring of tryptophan 18 of the protein N (Lcgault er cd..
1998).
Protcin affinity chromatography with the full length N protein demonstratsd a direct
interaction bctween N and thc E-coli elongation factor. NusA (Greenblatt and Li. 198 1 b).
Reccntly. a Lletailed delelion analysis of the N protein müpped the NusA-intcracring region of
N to rimino acids 34-47 (Mogridge et crl.. 1998a). A fragment of N ( N 1-47) inçluding this
retion of N ris well as the amino-terminal boxB-binding region of N. was able to promote
rcad-throrigh of a rho-independent terminator, but not as well as the full length N protein.
susgcsting tl-iiit a region important for maximal activity was missing (Mogridge ct al..
I998a ). In t'iict. affinity chromatography with other regions of N identified RNA
polynicrasc-binding regions in the carboxy-terminal region of N and in the aniino-terminal
47 miino iicids of N (Mogridge et al.. 199th). These studies confimed carlicr suggestions
that X intcracts directly with RNA polymerise (Ghysen and Pironio. 1972: Rccs et al.. 1996).
%us Factors
Hoht mutations that impaired the ability of wild type phage h. but not N-independcnt
mutant phage. to grow in E - d i led to the identification of Nus (A!-ltrilization substance)
protcins. TIiese rnoleculcs included NusA (Friedman. 197 1 ). NusB (keppel cr trl., 1971:
Friccimiin ct trl.. 1976), Nus€, or rîbosomal protein S IO. (Friedman et cd., 198 1 ). NusC. or the
p subunit of RNA polymemse (Georgopoulos, 197 1 b; Georgopoulos. 197 la). and NusD. or
termination factor rho (Brrichet er ni.. 1970: Simon et ni.. 1979). Confirmation of an
essential role for Nus proteins in antitermination was provided by subsequent irz r-ii-o and in
\.irro cxpcri ments as discussed txrlow.
Thc rrrrsA1 mutation was the first mis mutation to be identified. A rlrrs.4 1 strain
eshibits a strong temperature-sensitive phenotype, preventing J. growth at 4Z"C but not at
30°C ( Fricdnian. 197 1 : Friedman and Baron, 1974). Differences between the Strl~norr~il~~
gplri~~~irr-irrtrr NusA (NUSA'') and the Ecoli NusA (NUSA'*) enplain the inabilit). of h to grow
or the À 3i protein to function in Sairttoneli~r ryliimurilrrri (Baron et cri.. 1970: Friedman and
Baron. 1 974: Schauer er crl.. 1987). Sequence cornparisons and the construction of hybrid
NusA protcins then identi tied functional regions of NusA (Craven and Friedman. 199 I : Ito et
u/ . . 199 1 : Critven er rd.. 1994). NUSA" and NUSA" show high amino acid sequcnce
sin~ilrtrit>.. uith small regions of heterogeneity distributed throughout the proteins (Craven et
(ri.. 1991). The most striking difference between the proteins is located in the amino-terminal
one-third of the proteins and is called the 4-for-9 region. This region has fo~ir iimino acids in
the Ec-o/ i i-crsion and nine rimino acids in the S.rjphirn~trirrrri version of Nus.4 (Crriven et trl..
9 9 1. 1 nicrestingly. NusAh' can substitute for NUSA'" to siippon bacterial viiibi lity. biit i t
cannot support the growth of'h at any temperature (Baron er ai.. 1970: Friednian and Baron.
1974). Fiinhrirmore. a NUSA protein. called NusA 449. that \vas composed eniircly of NUSA"
csccpt tor thc 4-for-9 region. was able to support the action of h N (Craven et cil.. 1994).
Thus. the tCw differences between the two proteins may dctine regions important for
intcrxtion n-i th the antitermination complex.
nusA is an essential gene in E-coli. However, it is possible for the bacteria to survive
in the absence of rt functional NusA protein. but only when there is a mutation in the rho
termination factor (Zheng and Friedman, 1994). As NusA functions as a bacterial
elongation factor that enhances pausing and the efficiency of termination at certain sites
(Kung et cd.. 1975; Greenblatt et al., 1980). the connection between NusA and rho probably
has something to do with the postulated function of NusA in maintaining the tight coupling
between tnriscriptiona and translation (Ruteshouser and Richardson, 1989: Zheng and
Friedman. 1994).
A direct interaction between NusA and RNA polymerase has been demonstmted by
affinity chromatography (Greenblatt and Li. 198 la). In Chapter 2 of this thesis. 1 describe
deletion studies of NusA and the identification of two distinct RNA polyrnerase binding
regions. one in the arnino-terminal 137 amino acids and one in the carboxy-terminal 192
arnino acids. Other experiments further identified an interaction between the or subunit of
RNA pol y merase and the carboxy-terminal RNA polymerase-binding region ( Chapter 3 ).
This rcsult confirmed other in virro binding experiments that concluded that NusA interacts
directly with or. in addition to B and P' (Liu et al., 1996). I t was shown that the interaction
between a and NusA is important for the regulation of transcription since RNA-protein
cross-linking experiments revealed that a interacts with nascent RNA and this interaction is
abolishcd in the presence of NusA, which itself interacts with the RNA (Liu and Hanna.
1995b). The cross-linking of NusA to the nascent RNA was lost when a variant cx that
Iackcd its cntirc carboxy-terminal domain was used in cross-linking experiments.
Functionally. this truncated a interfered with the ability of NusA to enhance termination and
Q-mediated antiterrnination. but not N-mediated antitermination (Liu et cd.. 1996).
As rnentioned above. NusA also interacts directly with N (Greenblutt and Li, 198 I b).
.More speci tically. a carboxy-terminal region of NusA intsracts with m i n o acids 34-47 of N
(Chripter 3: .Mogridge er crl.. 199th). These results, in addition to those pertaining to the
RNA pol>.merase-NusA interaction (Chapter 2). form the basis of a mode1 for NusA function
that is discussed in Chapter 3 of this thesis.
Continued study of the relationships arnong NUSA" and NU SA^ and the hoxA
srqucnccs lCom 1, and Pz7 Ird to pnet ic evidence suggesting that NusA intcracts with bo.iï\.
The core 170x4 sequences from both phages are almost identical with one exccption: nirrL of
P X has thrcc thymines in positions 6-8 whereas both i z l r r i . and rzrrrR of 3c ha\.c turo thymines
and an adcnine in the same position. A mutation in A b a d that altcred the sequence T T A to
TTT <illoa*c.d NUSA" to support the growth of A. (Friedman and Olson. 1982).
Scqiience alignments identified S 1 and KH homology regions as two putative RNA-
binding rcgions in NusA (Gibson et cal.. 1993: Bycroft et c i l . . 1997). Other proteins that hatpe
S 1 and KH domains are able to associate with RNA and have both specific and non-specifk
RNA-binding properties. For example. the hnRNP K protsin. for which the KH domain has
bcrn rianicd. is the major oligo-C-binding protein in vertcbrates (Swanson and Dreyfuss.
1988: Sionii et al.. 1993). Vigilin is a protein made up of 14 KH domains that specifically
binds to thc 3' UTR of vigillotellin mRNA (Dodson and Shapiro. 1997). Whilc some isolated
KH don-iains can bind sonie sequences with higher affinity. thcy also have a Iouw aftinity for
other nucicic ricids. providing some evidence for their ability to associate ivith nasccnt RNA
(Dejgrirird and Leffers. 1996). These observations introduced the possibility that NusA might
bind \\pith h i ~ h affinity to certain specific sequences, Iikc the rurt site. yet also interact with
lotver iiftïnity with other RNA sequences.
The S 1 domain is named after nbosomal protein S 1. which has 6 of thesc RNA-
binding motifs (Subramanian, 1983). S 1. as part of the SOS subunit of the ribosonie. binds
pyrimidine-rich RNA sequences upstrerim of the ribosome binding site to fiacilitrite
translational initiation (Boni et al., 199 1). Thus, S 1 probably interacts with niliny sequences
with low al'finity. Howevrr. S 1 can select specific RNA ligands from a randorn pool of
RNAs [Ringquist et cd. . 1995). In fact, S 1 binds with higher affinity to À hosr\ RfvA since
mutations in h0 .d weaken the association of S 1 with ho-d-containing RNA (Mogridgs and
Greenbliitt. 1998).
Se~.eral lines of evidence suggest that the putative RNA-binding regions in NusA are
functional in RNA-binding. k mRNA was retained on a nitrocellulose filter by NusA.
iilthough t h i s interaction did not require the presence of bouî (Tsugawa et c i l . . 1985).
Protein-RKA cross-linking experiments within an elongation complex demonstrated that
NusA intcrricts with nasccnt RNAs greater than 10 nucleotides in length (Liu and Hanna.
1 9 9 5 ~ L~L I and Hanna. I99Sb). Interestingly. if NusA was not present. the RNA cross-linked
to thc cc-subunit of RNA polymerase. and the addition of NusA abolished this interaction.
Again. hoxA was not requircd for cross-linking.
Gel i-etardation assays usine purified proteins and a rlr tr site-containing RNA probc
providcd il1 1-itro evidencc for a specific NusA-nrtt site interaction. Wild typc NusA
supcrshil'tcti an N-nut site complex (Mogridge et al.. 1995). but this was blockrld by
mutations in certain nuclcotides in bo.\rA and hosB (Chaptcr 4; Mogridgc et ol.. 1995).
Furthcrmorc. mutation of'either a conserved arginine ( R 199). in the S 1 homology region of
NusA or thc tinsAl mutation located in the S 1 homology region. resulted in NusA proteins
that could not supershift the N-nrtt site complex (Chapter 4: Mogridge et cri.. 1995).
Interestingly. NusA 911. a NusA protein composed entirely of NU SA'^ except for the 4-for-9
region. does support the formation of this complex but faiIs to support the formation of a
compics additionalIy containing RNA polymerase, NusB. NusG and S 10 (Chripter 3). These
obssmations suggest that NusA interacts directly with RNA. a hypothesis thrit is further
esplorcd in Chapter 4 of this thesis.
NmB has apparent roles in both transcription and translation. The rrlrsB5 mutation
partirill>l rclicves polcuity suggesting that it may be involved in transcription termination
( W x d and Gottesman. 198 1 : Shmock et ai.. 1985a). The same mutation af'fccts the
hynthcsis of rRNA (Sharrock et al., 1985a). NusB has also been implicated in translational
clontrition bccause an IS 10 insertion in the rrrrsB gene reduces the peptide chain èlongation
rate by 30% t Taura et al.. 1992). Many rilrsB mutations exhibit the traditional Nus phenotype
( Kcppel cr r r i . . 1974: Friedman et ni.. 1976) and other nlrsB mutations suppress the defects in
mtitcrmin;ition by the h N protein of the nrcsAI and nrtsE7/ mutations (see hclou.: Friedman
and Baron. 1974; Friedman et ai.. 198 1) . /rl ritro confirmation of the direct in\.olvsment of
SusB in antitcrrnination by N was provided when extract made from iirrsB5 mutant cells that
ivas dckctii-s for antitermination was supplemented with wild type NusB to restore the
iibility of t hc extract to support N activity (Georgopoulos (II.. 1980; Ghosh iind Das. 1984:
Swindk et ~ r l . . 1988).
NLISB interacts dircctly with ribosomal protein S IO (i.e. NusE) (Mason cDr d. 1992a)
and othci- observations had outlined the importance of / 7 o ~ t A for NusB function in
antitcrrnination (Horwitz et tri.. 1987; Court et al., 1995). The discovery that 5 i i sB and S 10
togethcr bind the ho.rA RNA sequence from the E.coli rr-11 operons. but not thc closely related
onc from h. hclped support the hypothesis that NusB and S 10 bind h Uo-vA unstably and
require the presence of the other factors in the antiterrnination complex to maintain a stable
interaction (Nodweii and Greenblatt, 1993). In fact. NusB and S 10 associate stably with A
hox4 RNA only when N. NusA. NusG and RNA polyrnerase rire present (Mogridge rr til..
l998b i. inhibition of antitermination caused by the overproduction of bo-wl RNA was
restored by the overproduction of NusB suggesting that NusB interacts with 1 ~ 0 ~ 4 RNA
( Friedman c f ai . . IWO). A n N M R structure of NusB (Huenges et cil.. 1998) is consistent with
the possibility that an arginine-rich region of NusB may interact with Oo-r.4 RN.\.
Nrhsn ribosomal protein S 10 was identified as the NusE factor (Friedman et al..
198 1 ), i t \ v u not clear if S 10 was acting alone or whether the ribosome was :ilso involved in
antiterrnination. In fact. addition of purified S 10 or the 30s ribosomal subunit cornplements
Lin antitorrnination-defective extract made from a nrtsE71 mutant strain (Das cJr tri.. 1985:
Horwitz cr tr i . . 1987). While it is still formaily possible that the ribosome has some role in
the proccss. two observations suggest that ribosomes are not important for antiterrnination by
S. First. i r i i-itro antitermination by N can be reconstituted with S 10 in the absence of 30s
ubunits (Li d.. 1992). Second, translation is not necessary for antitermination in crude
systcrns i r i i-irro (Goda and Greenblatt. 1985) and does not overcome the antitcrmination
dcfcct of'1rrrsE71 (Warren and Das, 1984). The finding that S 10 also interacts with RNA
pol y nitxiw t'urther supports the idea that the stability of the processive antitei-niination
cornplcs ciopends on multiple protein-protein interactions (Mason and Greenbl~itt. 199 1 ).
Xcic~G was identiffcd initially as a Nus factor based on its ability to allow the
rcconstitcition of antitermination by N with purified components (Li et cil.. 1992).
Antiterniination assays using an E.coli S 100 extract as thc source of host factors for the
rcaction iveri- more efficient than reactions containing only purified N. RNA polyrnerase.
NusA. NusB and S IO. suggesting that an additional factor(s) was required (Li ~t (il.. 1992).
Accordingly. an activity purified to homogeneity from S 100 extract that allowcd efficient
antiterrnination by N was identified as NusG (Li er al.. 1991). Consistent with this. genetic
experiments identified a mutation in the r w G gene that suppressed the effects of the nusA I
and r r ~ r s E i l mutations on antiterrnination by N (Sullivan rf (11.. 1992). This suggested that
NusG is tr~ily an integral put of N-mediated antiterrnination. However. deplction of NusG
from ccI1s had no effect o n the ability of N to promote antiterrnination. where~ts termination
at a rho-dcpcndent terminator was compromised in this situation (Sullivan and Gottesman.
1992). Thcrefore. some other unknown factor may be able to substitute for NusG to support
antitermination by N in iSi\w. NusG binds RNA polyrnerase (Li rr ni.. 1992) and rho factor
(Li d.. 1993). itdding more interactions to the intricate web of protein-protcin interactions
uvithin the 9-moditïed transcription compiex. The interaction of NusG with rho is important
for rho-dependent termination. but may also be a key aspect of the ability of Y-modified
RNA p~l>~nicrase to transcribe through rho-dependent terminators (see below).
.Mutations that cause RNA polymerase to be resistant to the dmg rifanipicin map to
the rpoB gene that encodes the P subunit of RNA polymerase. Importantly. certain
ri hmpicin-rcsistant ( r i f ) mutations that alter the ability of RNA poiymerass to terminate at
rho-dcpcndcnt and rho-indcpendent terminators also affectcd N-mediated antitcrmination (Jin
c f (11.. 1988 1. These riP mutants either enhanced or suppresscd the phenotype of ri rutsA
mutant. siiggesting that the region of the subunit defined by the rif mutations is involved in
mtitermination and somehow affects the fiinctioning of NusA (Jin el al.. 1988)
Ttic Das and Greenblatt laboratories took advantage of the genetic data to define the
factors rcquired to form a stable antitermination complex capable of terminator read-through
itz i i t r n . Transcription reactions using S30 and S f O0 extracts as a source of host factors
showcd that antitermination by N did not occur with extracts prepared from r i t r s t î . rzitsB and
rrlrsE mutant strains (Dac and Wolska. 1984: Goda and Grcenblatt. 1985: Honvitz er al..
1987 ). B y n~onitoring the position of the elongation complex. Das and coworkers were able
to riscenain that N became a stable component of the complex only after RNA polymerase
had transcribed through the rlrtf site (Barik ut cd. , 1987). These results were contirmed and
esprinded L7y experiments tiom the Greenblritt laboratory in which elongation complexes
wertl puriticd by gel filtration from reactions containing wild type or mutant Nus factors and
progrümrncd with DNA containing no n u r site, a wiid type tzrtr site, or a mutant rnr t site
(Honvitz ~ j t C I / . . 1987). Although the incorporation of NusA into an elongation complex was
indepcnticnt of the mit site. presumably because it interacts directly with RNA polymerase.
the incorporation of N dcpended on the nir t site and the incorporation of NusB rcquired thc
prcsencc of wild type NusA. S 10 and Izo-rA (Horwitz et cil.. 1987). Thereforc. [lie üssembly
of a conipictc cornplex is highly cooperative.
Gcl mobility shift assays are another way of testing the importance of cach
antitcrrnination factor for the stability of the overall complex. A complex containing RNA
polynicrasc. N. the Nus factors and the n i t r site RNA remains intact in a non-dcnaturing
polyacnlliriiide gel (Mogridge et al.. 1995). However. mutations in RNA polyrnerase. NusA.
h0sA. or l,o.\rB that interfere with antitermination prevent the formation of a stable. Icw
mobility conipiex containing al1 of the factors (Mogridge er al.. 1995). Thesc esperirnents
also cietincd an order of assernbly of the complex where N bound first to the ttrrt site RNA.
follon-cd h!. NusA and RNA polymerase. This qua t ema l complex senred. in turn. as a
scriffold Ior the further assembiy of the remaining Nus factors. NusB. NusG. and S 10
(.Mogridge et al., 1995). Interestingly, the stability of this cornplex does not depend on ri
DN.4 template.
A mode1 of the processive N-modified elongation complex is shown in Fig. 4. The
modei is based on and summarizes extensive genetic and biochemical obser-vations regarding
the protein-protein and protein-RNA interactions within the processive amitermination
cornples. The dissociation constants of these interactions have been measured or estimated
(summarized in Greenblatt, 1992). Although most of the interactions are relatively weak the
complete complex is stable enough to remain associated with RNA polymerase for at least 7
kilobases. In the case of the rightward operon of phage k. the complex remains intact to the
Figure 4. The Complete N-modified Antitermination Complex The multiple protein-protein and protein-RNA interactions are decribed in the text.
Mechanism of Antitermination
Despite the requirement for the Nus factors and the mit site for antitermination irl
iYr-o. X c m cause RNA polymerase to ignore termination signals when present at high
concentration in an irz i.:irro transcription assay (Rees et d.. 1996). The addition of NusA and
the r z r r t site strengthens the ability of N to antiterminate. but only through a tsrniinator located
close to t hc lzilt site (Whalen et al., 1988; DeVito and Dac. 1993; Rees et cd. . 1996). For
processi\.e antitermination. the presence of dl of the Nus factors is required (Mason et ai..
I992b: DcVito and Das. 1994).
Tl~c above studies suggested that the Nus factors function primarily to provide
stability to the complex. while N provides the actual antitermination activity. N appears to
have ut Icast two functions in antitermination: first, N interricts directly with R-iA
polymerase and probably alters its ability to recognize termination signals; second. N
intcracts dircctly with NusA and suppresses the ability of NusA to enhance termination at :in
inrrinsic tci-minator (Mogridge et a l . , 1998ri). As well, since NusG interacts with rho factor,
and this interaction is important for rho-dependent termination. it is likely that the rho-NusG
intcraction tias been exploited by N to prevent termination rit rho-dependent terminators. In
I'act. a mutant rho factor. Rho026, prevents antitermination by N at high tempcrritures and
also cause3 the rho-NusG interaction to become temperature sensitive (Li el (11.. 1993).
These observations suggest that NusG rnay bind rho in the antitermination coniplex to inhibit
rho's tcrniination activity (Li et al.. 1993).
The exact mechanisms by which N interferes with activities of RNA polymerase and
NusA in tcrmination have not been defined. Some possible mechanisms are brised on the
reversa1 of key steps involved in tennination and include preventing the formation of pause
or torminator hairpins. blocking a critical site on RNA polymerase that interacts with pause
and terminator hairpins. stabilizing protein-RNA interactions within RNA polymerase. o r
preventing backsliding by RNA polymerase.
Other Antitermination Svstems
Scvcral additional prokaryotic antitermination systems have been identitied.
including Q-mcdiated antitcrrnination. rni antiterrnination. Bgl antiterrnination and prrt-
meditated antiterrnination. Study of these systems may help identify common fcatures o f the
antitermination process. Likewise. the differences may also provide interesting ciues to
csptliin i x i a t ions among the systems.
Q-mediated Antitermination
The Q protein is a product of the rightward early operon that rcquires the
fùnction of 3 for its expression. Q, in turn. is required for the expression of thc Iate genes
undcr thc control of the Iate promoter, PR'. Early investigations suggested that Q, like N.
reiieves polarity and functions as an antiterminator. The tirst in rirro evidcncc tor the
anritermination activity of Q was provided by transcription assays in which thc only protcins
present wcre Q. NusA and RNA polynierase. Read-through of terminators located
downstream of ;\. PR' was dependent on the presence of Q (Grayhack and Roberts. 1982).
Funhcrrnore. NusA stimulated this activity. suggesting that NusA is a true conlponent of this
second antiterminütion system in h (Grayhack and Roberts. 1982).
Tlic characteristics of Q-mediated antitermination di ffer from those of N-rnediated
antiterrriination in many ways. First. NusA seems to be the only host factor. othcr than RNA
polymerasc. that is involved in the Q system (Grayhack and Roberts. 1982). :Moreover. the
recognition site for Q protein. called yrlr (Q-iiRlization site) is found partly in the promoter
DNA. rathcr than entirely in the nascent RNA (Yang et cd.. 1987: Yarneli and Roberts.
1992). Q binds the DNA in the promoter and only recognizes RNA polymerasc when it has
paused ai a specific site downs t rem of the P,' promoter that is not found downstrearn of
most oihcr promoters (Grayhack et cil.. 1985; Yarnell and Roberts. 1992). Furthcrmore. the
proccss by \vhich Q interacts with RNA polymerase is unique to the system. RNA
polymcrasc must be paused naturally at position +16/+17 downstream from P,' in order for
Q to cngasc the transcription compiex and release ir from the pause site (Yarnell and
Roberts. 1992). This spcci tic pause is dependent on sequences in the initial transcribed
resion of thc promoter. particularly positions +2 and +6 in the non-templatc strand (Ring and
Robcns. 199.4). Interestingly. these two positions are within a region of high scquence
honiology to the 0 7 0 consensus - IO-binding site (Ring et cil.. 1996). Although the elongation
comples at + 16/+17 is h r enough downstream of the promoter to have released a"'. a"' is
still prescnt and necessary for the pause even though the a"' molecule has los[ contact wiih
the - 10 and -35 promoter scquences and the RNA polymerase tootprint is onIy 30-35 brise
pairs long ( Ring et rd.. 1996). Thus. a"' releases its contacts with the promoter only to rebind
iit ri similiir scquence jus1 downstream of + 1 to initiate the pause n e c e s s q for Q-function rit
+16/+17.
Thcre are some similatities between Q-mediated and N-mediated antitermination. As
is the case for N, RNA polymerase modified by Q becomes resistant to both rho-independent
and rho-dependent terminators, and this modification can persist through multiple
terminators. As well. both Q and N suppress pausing by RNA polymerase (Yang and
Roberts. 1989: Mason et ul.. 1992b) and this is likely to be related to the mçchanisms of
antitcrniination in both crises.
rrn Antitermination
Thc scven rm operons in E-cofi. which encode 16s. 23s. and 5s rRNAs and certain
tRNAs. arc subject to numerous forms of regulation. Some of these regulatory mechanisms
are discusscd elsewhere in this thesis introduction. Here. 1 will focus on the antitermination
system and discuss its similarities to the h. N-system.
Tcinscri ption and translation are generally coupled in prokaryotes. As ribosomes
transiarc thc nascent RNA close behind RNA polymerass. rho-dependent terminritors remain
maskcd and premature termination is avoided. However. these terminators bccome active if
thcrc is a disruption in the coupling and rho is able to gain access to the nsisccnt RNA
transcript. Xevertheless, tcrmination does not occur in the untranslated rni opcrons. The
csistcncc ot'an antitermination system was strongly sugpested by the finding that. despite the
inscnion of a transposable sequence known to cause polarity in other operons. \.en/ little
polarit~. n-as observed in 1-rtrC (Morgan. 1980). Subsequent experiments sitowcd that
transcription initiated from an rrri promoter. but not the lrc o r (rra promoters. was able to
continuc through a rho-dependent terminator (Holben and Morgan, 1984). Thih system
rcquircd wquences related to the À nrtt site. located both in the promoter/lcadcr regions and
the spacer regions of the rnz operons between the 16s and 23s genes. Although a hairpin
scqucncc supcrficially rescmbling boxB is present in these critical regions of thc rnz operons.
ri /ma- l ikc sequence is necessary and sufficient for antitermination in iilw and i t l i~itr-o.
However. this bo-rA-directed antitermination functions well only with rho-dependent
terminators. and not with rho-indepedent terminators (Berg et [ri.. 1989: Alhrechtsen et c d . ,
1990).
Bcciiuse of the sirnilritity in the sequence requirenients for antitermination in the r-t-tz
and  'I systems, it seemed likely that some of the protein components of N- and rm-
n~ediatcd antitermination systems would be shared. In support of this idea. overexpression of
the h r r l ~ r site RNA inhibits rRNA production in vivo (Sharrock et (11.. l985b 1. In addition.
specific mutations in NusA and NusB. but not S 10, had a deleterious effect on the expression
of'thc 1-1-11 operons (Sharrock et c d . . 198%). Furtherrnore. NusB and S 10 togcthcr bound rnr
ho.d but not lambda bo-VA. in gel mobility shift experiments (Nodwell and Greenblatt. 1993).
NusA hiis hcen implicated in the system by a mutation in the t z l t sA gene that iit'fkcts
iintitcrniination hl i7ii.o (Vogel and Jensen. 1997). Reconstitution of rni antitermination itl
\- i tr-o in rcactions containing E.coli extracts showed that NusB is absolutely required for
antitermination (Squires cr cri., 1993). Although the addition of NusA. NusG and S 10
irnprovcd tcrminator read-through in this system, it has not yet been determincd whether they
arc absolu tel y required for antitermination (Squires et c d - . 1993). Elongrition complexes
synthcsizing rRNA Nz i-itro have k e n shown to contain NusB and NusG (Li cl trl . . 1992).
put and nun in phage HK022
Thc coliphage HK022 has developed a system to prevent superinfection ot'a lysogen by
h. The HK022 encoded nun protein binds to the bmB portion of the h rrrlt site with an
aftlnity sin~ilar to that of N protein (Chattopadhyay et al.. I995b). [nstead of criusing
mtiterminrition. nun causes termination just downstream of the nrit sites (Robert et ni., 1987;
Roblcdo c r al.. 1990). The nun-termination system also sharcs some E-coli Factors with the
N-anti~crrnination systern. Mutatio~s in NusA, NusB and S IO rhat block antitermination by
N also block termination by nun (Robert et cd.. 1987; Robledo er cd. . 1991 ). In hct. these
mutations as well as othsr mutations in ho.tA, cause nun to act as an antitermination factor rit
the terminaior. tR,, but this alteration in function does not persist for terminaiors located
tiirther do~vnstream (Robledo et cd.. 1990: Baron and Weisberg. 1992). Thus. the nun and N
systenw .;hrirc components. yet differ in fundamental ways.
Thc nLin effect has been studied Nr iitro. Since one possible mechanism of
antiterniination is prevention of backsliding by RNA polymerase. i t is intrig~iing to note that
nun crin block elongation irz \.itro by the transcription complex without causing it to
dissociatc (Hung and Gottcsman, 1997). In this arrested complcx. RNA polynicrrisc retriins
çat:ilytic ;icti\.ity. More specificaily, the 3' OH terminal n~icleotide of the nasccnt transcript
ciin undcrp pyrophosphorolysis and restoration by the addition of nucieoside triphosphates
(Huns and Gottesrnan. 1997). However. only this terminal nucIeotide c m bc modified:
tùrthcr translocation of the complex, either forward or backwards. is prevcntcd by nun.
HK022 also has its own antitermination system. However. this system is uniq~ie in that no
phage-cncoded ant iterrninütion factor is required (Oberto et c d . . 1993). RNA polymerase
initiating from the rightward and leftward promoters becomes resistant to terminritors due to
the action of cis-acting RNA sites. called pur sites (polymcrase ittilization site). that increase
the rate of RNA polymerase elongation. probably through an ami-pause mechanism (Oberto
ef ai.. 1993: King et al.. 1996). This modification may not involve any of the Nus factors
that are important for h N-mediated antitermination and nun termination. since mutations in
the Nus kictors that prevent antitermination by N do not affect the ability of the prtr sites to
dircct tcrminator read-through (Oberto et cd.. 1993). In Frict. a search for host hctors
invol\.cd in rintitermination yielded 14 E-cnli mutants that prevented HK023 growth, al1
locatcd in one of three conserved cysteine residues thought to be a part of a zinc-binding
motif in thc p. subunit of RNA polymerase. encoded by rpoC (Clerget rr cri.. 1995). One of
these mutittions was further studied i i z vi\w and NI virro to show that the mutation specitïcaliy
decreriscd tcrminator read-tlirough (Clerget et al., 1995). The observation that riIl of the
mutants isolitted in the screen were able to support h growth suggests that the mechanisms of
h and HK022 antitermination are highly divergent (Clerget el al.. 1995).
Thc key to the mechanism of put-mediated antitermination is clearly the interriction
bet\vcen thc j m site RNA and RNA polymcrase. since a purified system containing the
tcn~platc. DNA. RNA polyrnerase and buffer allows terminator read-through ( King Cr cl/..
1996). Tliiis. a detailed analysis of the RNA structure of the put site may yicld important
information on the mechanism of antitermination in general. prrri. and prrrR arc about 70
nuclcotidcs long and have similar sequences, but are locatcd in different positions within
thcir respective operons (Obeno et al.. 1993: King er al.. 1996). The predictcd prrt site
sccondary structure was tested by exposurc to single- and double-strand-specik
ribonuclcascs and by testing the effects of mutations in predicted stem regions (King c.1 cd..
1996: Banik-Maiti et al.. 1997). The predicted functional structure consists of two hairpin
stems of \wying length. separated by a single nucleotide. There are interna1 bulges
associnted n-ith the stems that seem to be important for function. but the loops at the ends of
the stems arc not. Exactiy how an interaction between prrt RNA and RiVA polyrnerase
pre\.ents termination by the enzyme is not clear at this time.
Thesis Rationale
In this thesis. 1 have rinalyzed the structure/function relationship of the E-coli elongation
factor. NiisA. in chapter 2. 1 identify resions of NusA that interact with RNA polymerase.
thc 1, N protein and RNA. In chapter 3. 1 show that the inabiIity of full length NUSA to
intcract ~vith RNA in the absence of N is due to an inhibitory dornain located in the carbosy-
tcrminril rcgion of NusA. 1 also show that this inhibition of RNA-binding is rclieved by
interaction of' the carboxy-terminal inhibitory region of NusA with either N or the a subunit
of' R N A polyrnerase. In chapter 4.1 provide evidence that NusA interacts dircctly with the
I>o.vA and irrrerh-r portions of the rzrrt site. Finally, in chapter 5. I summarize niy results and
suggest :idditionai experiments to extend Our knowledge of NusA function.
Functional Im-partance of Reeions in E.coli Elongation Factor NusA that
lntcract with RNA wlvmerase. the Bacterio~hage h N rotei in. and RNA
A \wsion of this chapter has been accepted for publication in Molecular Microbiology.
1 pcrtorrncd riIl o f the expcriments presented in this chapter except those shown in Figure 2.
\\.hich \\*as done by R. Muhandiram. J. Li constmcted and purified the His,-tagged NusA
proteins.
Abstract
Thc association of the essential E.coli protein NUSA with RNA polyrnerase incrcases
pausing and the efficiency of terrnination at intrinsic terminators. NusA is also part of the N
protein-niodified antitermination complex phage that functions to prevent iranscriptional
tcrminarion. 1 have investigated the structure of NusA by using various deletion frligrnents of
NusA in ;i variety of in \lirro assays. Sequence and structural alignrnents have susgested that
NusA has both S 1 and KH homology regions that are thought to bind RNA. 1 show here that
the portion of NusA containing the S 1 and KH homology regions is important for NusA to
enhancc hoih termination and antitermination. There are two RNA polymerlisc-binding
regions in NusA. one in the amino-terminal 137 amino acids and the other in the carboxy-
tci-minal 264 arnino acids: only the amino-terminal RNA polymerase-binding rq ion provides
ri functioniil contact that enhances termination at an intrinsic terrninator or antitçrmination by
Y. Thc c;ii-bosy-terminal rq ion of NusA is also required for interaction with X and is
i ni ponant h r the formation of an N-NusA-riur site or N-NusA-RNA poly mcrtisc-riltr si te
cornples: thc instability ot'complexes lacking this carboxy-terminal region of NusA that
binds S and RNA polymcrase can k compensated by the presence of the addiiional E.coli
elongation kictors. NusB. NusG and ribosomd protein S 10.
Introduction
NLISA and the additional host proteins NusB, NusG and ribosomal protein S 10 are
importani for the N protein of bacteriophage k to modify RNA polymerase into a
termination-resistant state (Das and Wolska. 1984; Goda and Greenblatt. 1985: Horwitz et
(il.. 1 987: Schauer et al.. 1987). This modification requires a cis-acting elemcnt. called the
i z i l r sitc. that is composed of two elements. l)o.rA and boxB (Salstrom and Szybalski. 1978: de
Crombrugghe et al.. 1979: Olson et ni.. 1982). N. the host proteins. and r r w site RNA
assemble into a highly stable complex that associates with elongating RNA polymerase
(Barik trl.. 1987: Horwitz et al.. 1987; Mason et al., 1992b).
Gcnctic studies on antitermination by the IL N protein have suggested thrit NusA may
interxt u.ith the h 0 . a portion of the m i l site (Olson et cd.. 1982; Friedman and Olson. 1983:
Olson ci trl.. 1984). whercris sel mobility shift experiments have shown that n-iutations in
both /10.~4 and hoxB specitlcally affect the interaction of NusA with an N-rrrrr site complex
(Mogridgc c t cil.. 1995: Chripter 4). Protein-RNA cross-linking data have shown that NusA
also inicracts directly with the nascent single-stranded RNA in a transcription complex (Liu
and Hanna. 1995b: Liu and Hanna. 1995a). In this contest. however. ho.d is not required for
the NusA-RNA interaction.
Consistent with thc evidence that NusA may interact with RNA. seqiience and
?;tructuraI alignments ha\*c indicated that NusA has S 1 and KH homology regions. both of
~vhich arc thought to interact with RNA (Gibson er al., 1993: Bycroft e t cil.. 1997).
Rclativcly little is known about the S 1 and KH homology regions in NusA. bcit .;orne
niutations in the S 1 homolo_oy region of NusA prevent h. growth without aftkcting the
viability of Ecoli (Y. Zhou and D. Friedman, unpublished data; Friedman. 197 1 ). These
mutations ol'ten affect the ability of NusA to associate with an N-nrtr site RNA complex in
oeI rnobility shift experiments, but do not directly affect the interaction between NusA and N C
(Chapter 4: T. Mah, Y. Zhou, N. Yu, J. Mogridge, E. Olsen. J. Greenbtatt and D. Friedman.
unpublishcd data; Mogridge et al.. 1995).
Othcr studies have focused on the interaction of NusA with RNA polymerrise. The
core R N A polymerase enzyme consists of essential a, P and p' subunits and one non-
essential subunit, o (Burgess et al.. 1969: Gentry et czl.. 1991 ). Genetic experiments have
suggested rhere nmy be interactions of NusA with P, P' and IX (Jin er d.. 1988: Ito et cil . .
199 1 : Ito and Nakamura. 1993: Ito and Nakamura, 1996: Schriuer et cil . . 1996 1. More
recently. i t \vas shown that NusA can bind directly to the carboxy-terminal domain (CTD) of
a. and possibly to P and P'. but not to the amino-terminal domain (NTD) of a (Liu ut cd..
1996). I t hiis aiso been suggested that a direct interaction between the a-CTD and NusA is
important for NusA's effects on pausing and termination. but not N-mediated antiterrnination
(Liu er trl . . 19%).
1 have taken a systematic approach in an attempt to assign functions to the various
regions ot' NiisA. By making deletions at putative domain boundriries and rissriying for
cfkcts on fiinction, 1 was able to identify two RNA po1ymeril.s~-binding regions. one in the
miino-tcrriiinal region and rinother in the carboxy-terminal region of NusA. 1 also found that
thc hinding of NusA to N rcquires amino acids in the carboxy-terminal region of NusA.
intcrestingly. Ioss of the crirboxy-terminal RNA polymerase-binding region docs not have an
cfkct on termination or antitermination by N. Similarly, loss of the N-binding region does
not hlit-c :in effect on antitermination. However. loss of the S 1 and KH homology regions or
t hc mi no-tcrrninril RNA polymerase-binding region abolishes NusA's ability to influence
cither ol'tticse processes. Surprisingly. the loss of the amino-terminal RNA polymerase-
binding rcgion of NUSA does not impair NusA's ability to assemble into a stable complex
containing hi, NusB. NusG. S 10, RNA polymerase and the rir t t site RNA. indicritint that
interaction of this region of NusA with RNA polymerase may have a direct role in both
termination and antiterrnination.
Results
Production and characterization of NusA fragments
Ntr>.A binds to the core component of RNA polymerase in order to modulate
transcription (Greenblatt and Li, 198 la). A schematic illustrating some of the putative
domains of NusA is shown in Fig. ia. These regions were assigned on the biisis of
compariscins with other proteins containing sirnilar domains or on the basis 01' scquencc
alignnients with NusA proteins from other organisms. Because of this. struct~irril analysis
must still bc done to confirm the exact domain boundaries of NusA. The SI and KH
homolop rcgions are putative RNA-binding domains. and the regions designated AR 1 and
AR2 are 50 amino acid repeat sequences which are 39% identical and 54% siniilar and
contain a high percentage of acidic residues (Gibson et ai.. 1993: Craven c r i . . 1993: Bycroft
el c d . . 1997 1. In order to define structure-function relationships within NusA. J . Li used these
p~itati\-c domain boundaries to make various polyhistidine-tagged deletion constructs. The
rcsulting lt-qments of NusA were then overproduced in Eco[!' and puritled to near-
homogencity by chromatography on a nickel chelate resin (see Experirnental Procedures).
Al1 thc Ni:sA fragments were highly soluble in native conditions.
1 x-talyzed al1 of thc purified NusA t'ragments by circular dichroism iCD)
spcctroscopy and some of the results are shown in Fig. 1 b. With the exception of NusA
( 1 32-240). rcpresenting the isolated S 1 hornology region. thcre were obvious niininia at
222nni ancl 208nm. indicatine the presence of a-helices. I n Fie. Ic are listecl the CTc a-helix
content 01'cach NusA fragment and, for comparison, the C/c a-helix predicted by the PHDsec
sècondary structure prediction program (Rost and Sander. 1993; Rost and Sander. 1994).
Figure 1: Circular dichroism anaiysis of NusA deletion constructs. (a) Predicted domain architecture of NusA. (b) CD analysis of NusA deletion constructs. Wavelength scans were performed on selected NusA deletion constructs from 300 nrn to 200 nm. ( c ) Cornparison of predicted 5% helix with actual % helix. Predicted secondary structure was generated by the PHDsec program. The values in the table for CTc helix were calculated using the following equation: ([8222] + 2340)/30,30 (Chen er al., 1972).
On the basis of these results. 1 think it is unlikely that the isolated S 1 homoiogy
region in NusA (132-240) is properly folded. As well, because the 15% a-helical content of
NusA ( 1-2-10} was much lower than the 3 1% predicted by the PHDsec program and much
lower than the 30% that would be predicted by cornparison with the actual a-helical contents
of NiisA ( 1-3-18), NusA ( 1 - 137) and NusA (232-348). it is probable that the S 1 homoloev b-
region in NLISA ( 1-2W) is also not properly folded. This conclusion was confirmrd by NMR
anülysis. as shown in Fig. 3. For this expriment. NusA ( 1-240) was labeled \vith "N and a
two-dinlensional ' H-"N quantum correlation (HSQC) spectrum was generated r Kay er <il..
1991: Zhüng et rd. . 1994). The lack of dispersion of the resonances in the spcctrum strongly
suggestcd that NusA ( 1-2-10) is improperly folded and highly aggregated. The likely lack of
proper folding was taken into account when interpreting the negative results dcscnbed below
~vith NusA ( 1-210) and NusA ( 132-240)-
In contrast. CD analysis showed that the amino-terminal fragment. NusA ( 1 - 137). has
high a-licliçal content. similar to what is predicted. As well. this fragment was able to bind
RNA polynierase (see below). In C D experiments, NusA ( 1 - 137) showed coopcrritive and
i-cversihle niclting with a T,, of approximritely 50°C (data not shown). providing strong
cvidcnci: !or an independcntly folded domain. This conclusion was confirmed by analyzing
ilic HSQC spcctrum of "N-liibeled NusA ( 1-137). as shown in Fig.2b. In this case. the
~cncr~ i l ly p o d dispersion of the resonances was indicative of a Iiighiy ordercd. lolded b
domriin.
By comparing the 33% a-helical content of the active fragment NusA ( 1-348) (see
below ) and rhc 39% helicai content of NusA ( 1 - 137), it can be predicted that NusA ( 132-348)
u.ould bc about 29% a-heiix. which is s imi lx to the actual value of 28% (Fig. lc). This
suggcstcd thrit the S 1 and KH homology regions in NusA ( 132-348) should bc lolded for the
most part. Again. this conclusion was verified by NMR f Fig. 2c). In this casc. the good
dispersion of the resonances in the HSQC spectrum suggested ttiat NusA ( 132-348) also has
ri hiehly ordered stucture. but the variation in signal intensity was suggestive of
conformational heterogeneity. The likely presence in this NusA fragment of thrce domains.
one S 1 homology region and two KH homology regions (see Fig. la), could generate such
conformational heterogeneity if one for more of the domains has freedom of motion with
respect to the others. The longer active fragments, NusA ( 1-3 16) and NusA ( 1-195). and the
niulti-domiiin inactive fragments, NusA ( 132-4 16) and NusA ( 132495). which can intetact
with both N and with N-rzrrr site complexes (see below), are also likely CO be tolded- I have
made the rissumption that these fragments of NusA, as well as NusA ( 1 - 137 and NusA ( 132-
348). are properly folded when interpreting the results of some of the experimcnts that are
describcd hclow.
Two RNA polvmerase-bindine - reeions in NusA
To dctermine which part of NusA is required for the NusA-RNA polyrnerase
interaction. çxboxy-terminal deletion mutants of NusA with His, tags at the amino-terminus
were L I S C ~ a s column ligands in affinity chromatography esperiments (Fig. 3a). Ecoli extract
containing additional RNA polymerase core component (Fig. 3a. lane 2 ) was passed over the
columns. The core polymerase subunits. P. p' and a, bound spccificall y to riIl of the delction
constructs that were tested (Fig. 3a. lanes 3-8), over and above the background binding to the
Yi-iigarosc column matrix (Fig. 3a. Iüne 3). None of the NusA fragments had acquired an
ribility to bind proteins non-specificaily. because few, if any. of the other protcins in the
Ecoli cxtriict bound exclusively to any of the NusA columns (Fig. 3a. compare lane 3 with
Iiincs 4-81. The srnallest NusA fragment that bound RNA polymerase, NusA ( 1 - 137) (Fig.
3a. lanc 4 1. corresponds to a moderately a-helical amino-terminal domain of NusA that folds
indcpcndcntly (see Figs. 1 b and 2b) and had not yet been assigned a function.
Figure 3: T~vo RNA polymerase binding-regions in NusA. (a ) NusA ( 1 - 137 binds RNA pnIymerase. E. coli extract containing additional RNA polymerase corc enzyme (lane 2; RNA polymerase is shown in lane 1 ) was passed over columns containing Ni-agarose (lane 3), 1.3 m@mL His6-tagged NusA (lane 8) or \.arious His6-tagged NusA dcletion mutants (lanes 4-7). The concentration of cach NusA delction mutant on the column was adjusted so that each had the same molar concentration ris the full length NusA. Bound proteins were eluted with buffer containing I hq NaCl, subjected to SDS-PAGE and stained with silver. (b) NusA (232395) binds RNA polymcrase. E - d i extract containing additional RNA polymerasc core enzyme (lane 1 ) was passed over columns containing Ni-agarosc ( lane 2). 1.3 rng/mL His6-tagged NusA (lane 6) o r equivalent molar concentrations of various His6-tagged NusA deletion mutants (lanes 3-5). Bound proteins were cluted with buffer containing 1 M NaCl, subjected to SDS-PAGE and stained with siiver.
Since direct binding experiments (Liu et al.. 1996) and NusA-RNA polymerase
crosslinking experirnents (J . Li and J. Greenblatt. unpubIished data) have shown that NusA
may be able to bind to more than one subunit of RNA polymerase. 1 reasoned that there could
be anothcr RNA polymerase-binding site on NusA. Therefore. His,-tagged constmcts that
lacked either the first 136 amino acids of NusA (data not shown) or its first 232 amino acids
(Fig. 3b) n-ere coupIed to Ni-agarose and used as affinity chromatography ligands. In this
case. thc p. p. and a subunits of RNA polymerase were specificaIly retained on a column
containing SuIl length NusA (Fig. 3b, lane 6 ) . as well as on a colurnn containin_r NusA (232-
195) (Fig. 3b. lane 5 ) . but deIetion of the carboxy-terminal 79 amino acids in NusA (232-
4 16) rcsultcd in the loss of RNA polymerase binding (Fig. 3b. lane 4). Similarly. a NusA
protcin bcginning at amino acid 132 would not bind RNA polymerase when thé carboxy-
terminal 79 amino acids of NusA were deleted (data not shown). Thus. therc are two RNA
polymcrase-binding regions in NusA, one in the amino-terminal 1 - 137 amino acids and
another in thé carboxy-terminal 264-amino acids. These conclusions were conlirmed by two
additional csperiments showing that both highly purified RNA polymerase corc enzyme and
the endogenous RNA polymerase in a cmde extract bind specifically to colunms containing
imrnobilizcd NusA ( 1 - 137 1, NusA f 232495). and NusA ( 1 -495) (data not shoum 1.
Therct'ore. YusA ( 1-1 37) and NusA (332-495) can bind directly and independcntly to RNA
pol yrncrrise.
Onlv the arnino-terminal RNA ~olvmerase-bindin~ region of NusA is necessart. for
cnhanccnicnt of termination at an intrinsic terrninator
SiiisA increases the termination efficiency of RNA polymerase at many intrinsic
tcrminiitors (Greenblatt er tri.. t 98 1 : Ward and Gottesman. 198 1 : Farnham ci (11. . 1982;
Schniidt and Chamberlin. 1987). It was therefore of interest to detemine thc impact of the
loss of one or other of the RNA polymerase-binding regions on NusA's function in
% readthrough 43 42 48 27 22 18
% readthrough 40 19 42 38 45 46
Figure 4: Regions of NusA important for enhancement of termination irl i.irro. ( a ) Tlic carboxy-terminal RNA polymerase-binding region of NusA is not necessary for termination enhancement. (b) The amino-terminal RNA polymerase binding-region of NusA is necessary for termination enhancement. Transcription reiictions containing 25 nM RNA polyrnerase and 50 nM NusA. or various NusA dcletion mutants (as indicated). were electrophoresed on 6M urea 3% polyacryllimide gels. dried and exposed to film. Positions of the terminated and run-off transcripts are indicated.
terrnination. To this end. il1 i*ifro transcription was pexforrned on a template containing a
promoter. a wild type n w site, and the simple h terminator. tR' (Fig. 4a). When no NusA
was added. RNA polymerase alone gave 43% readthrough o f the tR' terminator. NusA ( 1-
495) substantiaily decreased the amount of readthrough to 18%. In contrkst to this effect, the
addition of XUSA ( 1- 137) o r NusA ( 1-240) had no effect on the efficiency of termination.
Surprisingly. even though NusA (1-348) and NusA ( 1 3 1 6 ) had lost the ability ro bind RNA
polymeriisè via their carboxy-terminal RNA polymerase-binding regions. both proteins
cnhrinccd termination alrnost as well as the full length NusA protein- Thus, the presence of
the carboxy-terminal RNA polymerase-binding region of NusA is not requircd !or
cnhanccnicnt of terrnination. The abiiity of NusA (1-137) to bind RNA polymçrase and the
evidencc from CD spectroscopy that this fragment of NusA is folded (see abovc). combined
ivi th thc iniibility of NusA ( 1 - 137) and NusA ( 1-240) to enhance termination. indicated that
the region containing the S 1 and KH homology regions (amino acids 132-3481 is required for
cnhanccmcnt of termination. Since the S 1 homology region in NusA (1-240) is unlikely to
be foldeci (sce above). i t is still unclear which of the S 1 and KH homology rcgions are
indi\.idually required for NusA to enhance terrnination.
The Fict that NusA constructs containing only the amino-terminal RiVA polymerase-
binding rcg ion and the S 1 and KH homology regions were able to enhance termination
suggested t hiit the amino-terminal RNA polymerase-binding reg ion might bc \,c.ry important
for this proccss. To test this idea, in vitro transcription was carried out using fnrtr NusA
constructs ol'various lengths that lacked this region (Fig. 4b). and none was able to enhance
tcrmination. Since CD and NMR experiments on NusA ( 132-348). which cannot enhancc
terminrition. had indicated that this portion of NusA is folded o n its own. thesc I-csults
showcd thiit the amino-terminal RNA polymerase-binding region is necessary for NusA to
enhancc tcrmination and cannot be compensated by the carboxy-terminal RNA polymerise-
binding region tliat is present in NusA ( 132-495).
ft eluate
Figure 5: N hinds a carboxy-terminal region of NusA. A mixture of His6-tagged NusA and His6-tagged NusA deletion mutants (as indicated. Iane 1 ) was passed o \ w affinity columns coniaining 2 mg/mL GST (lanes 2 and 4) or 0.5 m@mL GST-N ilanes 3 and 5). The flow through (lanes 2 and 3) and the 1 M NaCl duate ( Irines 4 and 5 ) fractions were subjected to SDS-PAGE and stained with silver.
A carboxv-reminal region of NusA is necessarv for the binding of N
NusA a h binds the bacteriophage N protein (Greenblatt and Li. 198 i b) and these
proteini, can together associate with RNA polymerase to prevent termination of transcription
when a r i u r site is present (Whalen et al.. 1988; DeVito and Das. 1994: Mogridge et d.. 1995:
Rees el ( J I . . i996). In order to determine which part of NusA is required for ri direct
intcrricrion with N. GST-N was used as an affinity ligand in binding experirnents (Fig. 5) . A
mixture of His,-tagged full length NusA ( 1-495) and four carboxy-terminally deleted
proteins. NusA ( 1 - 137). NusA ( 1-210). NusA ( 1-348) and NusA ( 1-4 16). as shown in lanc 1.
\vas passcd over a GST control column and a GST-N column with sufficient capacity to bind
a11 the NusA fragments. Al1 of the proteins flowed through the GST column (Fig. 5. lane 2 )
and \ircrc absent from the salt eluate of this column (Fig. 5. lane 4). In contrast. the flow-
through ironi the GST-N column contained only the three smallest proteins. NiisA ( 1 - 137).
NusA ( i -?-!O) and NusA ( i -348) (Fig. S. Iane 3). whereas the salt eluate from this column
containcd significant amounts of only full length NusA and the deletion construct that had
the smatlcst carboxy-terminal truncation. NusA ( 1 - 4 16) (Fig. 5. lane 5). Sincc NusA ( 1 - 3 4 )
is actiw in tcrmination and antitermination assays (see above and below). i t must be properly
hldcd. Thcrcfore. these rcsults indicated that the N-binding ability of NusA rcquires a
crirbosy-tcrniinal portion of NusA. but the extreme carbosy-terminal 80 amino x i d s are not
rcquircd t'or this interaction.
The amino-terminal RNA polyrnerase-binding - region of NusA is essential for enhancino
antitcrniination bv N
Nu'sA enhances an intrinsic ability of N to antiterminate transcription (Whalen c.1 crl..
1988: DcVito and Das. 1994; Rees et d.. 1996). Therefore. iu iitro transcription was
perfornicd in order to assess the effects of progressive carboxy-terminal deletions in NusA on
% readthrough 45 21 65 63 81 78 80
% readthrough 46 ô8 86 57 58 58 51
Figure 6: Regions of NusA necessq for enhancement of antitermination by N. ( a ) The carboxy-terminal N-binding region of NusA is not necessary for cnhancement for antitermination by N. (b) The amino-terminal RNA polymerase binding-region of NusA is essential for enhancement of antitemination by N. Transcription reactions containing 25 nM RNA polymerase. 100 nM N and 50 nM NusA or various NusA deletion mutants (as indicated) were electrophoresed on 6 M urtla 4% polyacrylamide gels, dried and exposed to film. Positions of the tcrn~inated and mn-off transcripts are indicated.
the ability of NusA to influence antitermination (Fig. 6). Addition of N protein alone to a
rcaction increased the amount of readthrough from 46% to 68% (Fig. 6b). When NusA wa_s
added to a rcaction containing N and RNA polymerase, the readthrough was fiirther
increased to 86%. Addition o f NusA ( 1 - 137) or NusA ( 1-240) to the reaction had no effect
on the ability of N to antiterminate (Fig. 6a). Surprisingly. however. NusA 1 1-318). which
had lost its ribility to bind directly to N. enhanced the effect of N on readthough of the tR'
tcrminator as weIl as the two N-binding constmcts, NusA ( 1 3 16) and NusA ( 1495) ( Fig.
6a). Conibincd with CD and NMR evidence that NusA ( 1 - 137) is folded and other evidence
that it binds RNA polymerase. the inability of NusA ( 1-240) and NusA ( 1 - 137 ) to enhance
mtitermination by N indicated that the region containing the KH and S 1 homology regions
(amino x i d s 132-348) must be citical for this process.
Espcriments were also carried out to assess the effect of deleting the amino-terminal
RNA poiynierase-binding region of NusA on antitermination by N (Fig. 6b). Loss of the
amino-icrniinal RNA polymerase-binding region of NusA in NusA ( 132-3481. NusA ( 132-
4 16) and NusA ( 132-195 ) greatly affected the ability of NusA to enhance antitcrmination by
N. Thc Ic\*els of readthrough observed wiih these amino-tcrminally uuncatcd NusA
constructs n w e not increased beyond the ievel that N could promote on its own. In fact.
therc \{,as xturilly less readthrough, suggesting that these mutant NusA proteins may be
intcrkring n.ith antiterrnination by N \ria an unknown mechanism. Thus. evcn the amino-
tci-niinally clcleted N-binding constructs. NusA ( 132-4 16). which c m interact with an N-nul
site coniplss (see below). and NusA ( 132495). which can cven assemble into complexes
containing R N A polymerase (see below). were unable to influence antitermination.
suggecsting that the amino-terminal RNA polyrnerase-binding region of NusA is critical for
this proccss.
Effects of NusA deletions on the formation of N-NusA-n~tr site com~lexes
Gel niobility shift experiments c m be used to assess the interaction of N. the E - c d i
Nus hctors. and RNA polymerase with "P-labeled mir site-containing RNA (Mogridgr et of..
1995 1 . In order to assess the importance of the various NusA domains on the binding of
NusA to Y-rrrrt site complexes. gel shift experiments were done with N and wrious NusA
dcletion constructs (Fig. 7). N protein alone is sufficient to bind and retard the mobility of
R N A contiiining a wild type nrtt site (Fig. 7. lanes 1 and 2 ) . whereas full length NusA cannot
sh i t i thc RSA on its own fiMogridge et cd . . 1995). When fu l l length NusA was added to the
reaction containing N. the RNA was supershifted from the N-n~tr site comples ( Fig- 7. fanes
7. 12. and 16 ). NusA ( 1-4 16). which can also bind N directly (see Fie. 5). u-ris rilso capable
of supershitiing the N-RNA complex (Fig 7. lane 6). In this case. the supershifted complcs
had lou.er riiobility. perhaps because deleting the last 80 amino acids of NusA altcrs its
conformation or causes it to dimerize. As cxpected, the dcletsd, non-hnctiond NusA
protcins tha~ were not able to bind N, namely NusA (1-137) and NusA ( 1-2-40). \vere not able
to supcrshift the N-RNA coniplex (Fig. 7. Irines 3 and 4). and cven NusA ( 1 --348). which
supports mtitermination but cannot bind N. interacted more weakly with thc complex (Fig. 7.
Imc 5 1. Sonc of the NusA fragments except NusA ( 1-416) can directly bind thc RNA in the
dxcncc of' Y (Chripter 3). Thus. only NusA constructs that c m bind N can intcrict with an
3-jrrrt 'rite complex strongly enough to create a supershift in a non-denaturing gel. Because
thc carbox!.-terminal N-binding region of NusA is important for the supershifi. I could not
dctcrminc \t-hethcr the KH and S 1 homolo~y regions of NusA are also neccssary for complex
tormat ion.
To ;isscss the effcct of progressive rimino-terminal deletions on the ability of NusA to
wpcrshil't a n Y-nrrt site compkx. gel mobility shift experiments were done ueith NusA
constriicl'r truncated eithcr at amino acid 132 or at amino acid 232, and thus n~issing cither
the rimino-terminai RNA polymerase-binding region (amino acids 1- 137) or this region plus
Free Probe ---)
N NusA
NusA (1-137)
NusA (1-240)
NusA (1 -348)
NUSA (1-416) NUSA (1 32-240) NUSA (1 32-348) N USA (1 32-41 6) NusA (1 32-495) NUSA (232-348)
NusA ((232-41 6) NusA (232-495)
Figure 7 (preceding page): The effects of NusA deletions on the formation of N-NusA-mit
site complexes. Reactions containing "P-labelled mir site RNA. 1.5 pM N. and 1 .O p M
NusA or various NusA deletion mutants (as indicated) were electrophoresed on 7.5% non-
denaturing gels. dried and exposed to film. The results from three sepante gels were
combincd. but control Ianes 1 and 2. originally present in al1 the gels. are sholvn only once.
Figure 8 ( following pagc): The effects of NusA deletions on the formation ot'complete
complcscs containins N. the Nus factors and RNA polymerase. (a) The portion of NusA
containing the KH and S 1 homology regions is important for the formation of the complete
complcs. t b) Either R N A polymerase binding-region is sufticient to support complete
complcs liirniation. Reactions containing "P-labelled riir! site RNA and various
combinations of 500 nM N. 50 nM NusB. 50 nM NusG. 50 nM S 10.25 nM RNA
polynic~isc and 100 nM NusA or NusA deletion mutants (as indicated) werc electrophoresed
on 5% non-cicnaturing gels. dried and exposed to film.
\ RNA polymerase
R N A P - + + + + + + + + + + + + + + + N - - + + + + + + + + + +
e/ws - - - - + + + + +
NusA - 1-137 -240) * 1-495 *
1 2 3 4 5 6 - 7 8 -- 9 10 11 12 13 14 15 16 17
Compkte
N-NUSA-RNA polyrnembe
RNA polymerase
RNAP - + + + + + + + + + + + + + + + + N - - + + - + + + + + + + +
B/G/S - + + + - - - + u t - - NUSA 1 32-240 1 32-348 13241 6 132495 1495
the S 1 homology region (arnino acids 136-2331. NusA ( 1324 16) and NusA ( 1 32-495) were
able to support the formation of an N-NusA-nrrr site complex (Fig. 7. lanes 10 and 1 1 ).
These proteins retained the S 1 and KH homology regions and the N-binding region of NusA.
Thenfore. the amino-terminal RNA polymerase-binding region is not necessan for binding
to an N-rzlrr site complex even though it is necessary for NusA to enhance antitèrrnination.
As before. funher carboxy-terminal deletions to amino acid 240 or 348. which eliminated the
N-binding site of NusA, reduced or eliminated the ability of NusA to join the N-rutf site
complcx (Fis. 7. lanes 8 and 9). None of the constructs rnissing the S 1 homology region. as
well as the amino-terminal RNA poiymerase-binding region. namely NusA (232-318). NusA
(132-4 16 ) and NusA (232495). could interact with the N-nrtt site complex ( Fig. 7. lanes 1 3-
15). Sincc CD experiments indicated that the KH homology regions in thess fragments were
likely to be folded. and since constructs containing the S 1 homology region and extending to
amino x i d -il 6 or 495 were able to support complex formation (Fig. 7, lanes IO and 1 1 ). this
rcsult suggcsted that the S 1 homology region is required for interaction with thé N-nrtt site
complex. This conclusion would be consistent with the earlier observation that the rirtsAI
mutation in the S 1 homology region prevents NusA frorn binding to the N-riur complex
( Mogridgc cr (11.. 1995). Thus. at least the S 1 hornology region of NusA and an N-binding
region betn-ccn amino acids 348 and 4 16 are required in order for NusA to associate strongly
with an S - r i r r r site complss.
Effects 01' XusA deletions on the formation of com~lete comolexes containinc - N, the Nus
factors. ancf RNA ~olvrnerrise
A KUSA mutant truncated at arnino acid 343. which lacks the N-binding site of NusA.
is partial 1 y functional for antitermination irr vivo, and somewhat temperature-sensitive for
Ecoli growth (Tsugawa al., 1988). suggesting that most of the wild type fùnction is
retained in this protein. To investigate whether the presence of the remaining Nus factors.
NusB. I\iusG. and ribosomal protein S 10. as well as RNA polymerase, could somehow
stabilize the association of such a truncated NusA molecule with the N-rrlrr site complex. 1
performcd gel mobility shift experiments in the presence of al1 of the Nus factors, RNA
polymerasc and RNA containing a wild type mit site (Fie. 8a; Mogridge er al.. 1995). Wild
type NusX supports the formation of a complete, lower mobility compies containing N. al1 of
the Nus frictors. and RNA polymerase (Fit . 8a, compare lane 17 with lanes 15 and 16;
,Mogridgc cf trl.. 1995). Thc low mobility complex was still formed with NusA ( 1-4 16).
which lacks the carboxy-terminal RNA polymerase-binding reeion but still retains the
carboxy-terminal N-binding region and the amino-terminal RNA polyrnerase-binding region
(Fig. Sa. lanc 14). Interestingly. although NusA (1-348) seemcd unable to support formation
of the unstable N-NusA-RNA polymerase-riut site complex (Fit. 8a. lane 10) that was
observcd nith NusA ( 1 -4 16) (Fig. 8a. lane 13) or full length NusA (Fig. 83. Ianc 16). it was
ablc to slipport complete complex formation (Fig. 8a. lane 1 1 ). although not as eft'iciently as
NusA ( 1-495) or NusA ( 1-4 16) (Fig. 8a, compare lane 1 I with lanes 14 and 171. This result
correlatcd ~vtlll with the NI ~ i ~ w data (Tsugawa et czl.. 1988) and suggests that thc additional
stabilit~. pro~ided by NusB. NusG and S 10 can partially compensate for instribility caused by
thc loss of the N-binding and RNA polymerase-binding regions in the carbosy-terminal
region of KUSA. The shortcr NusA deletion constructs. NusA ( I - 137) and NusA ( 1-240).
wcre not able to support complete complex formation (Fig. 8a. lanes 5 and 8). indicating t h a ~
the portion of KusA containing the KH and S 1 homology regions is important for complete
comples formation.
To test directly whcther the amino-terminal RNA polymerase-binding rcgion is
important for forrning a low mobility complex containing RNA polymerase. 1 ~ised NusA
constructs that lack this region in gel rnobility shift experiments with N. RNA polymerase
and the rcst of the Nus factors (Fig. 8b). The constnicts that hcked the carboxy-terminai
RNA polyrnerase-binding region, as well as the amino-terminal RNA polynierase-binding
region. XusA ( 132-240). NusA (132-348) and NusA (1324 16). were unable to support the
formation of the complete complex (Fig Sb, lanes 5. 8 and 1 1 ). [nterestin_ol\.. the presence of
the carbosy-terminal RNA polymerase-binding region in NusA ( 132395) conipensated for
the loss of the arnino-terminal RNA polymerase-binding region such that a lotv rnobility
cornples \tpris formed (Fig. 8b. compare lanes 14 and 17) whose stability was increased by
KusB. SusG and SI0 (Fig. 8b, compare lanes 13 and 14). Thercfore. the prescnce of either
RNA polymcrase-bindincoe-in region is sufficient to support formation of a low rnobility stable
cornplcs c\.cn though only the amino-terminal RNA poIymerase-binding region is needed for
NusA to enhance antiterminrition (Fig. 6 ) -
Discussion
1 L I W ~ deletion constructs of NusA in various assays to identify hnctiontil regions of
the protcin (see Fig. 9). Ttvo RNA polymerase-binding regions were identitlèd by affinity
chrormtography: one was localized to amino acids 1- 137 of NusA and appears to be a folded
domain: the other in amino acids 232495 was eliminated by deleting amino ricids 4 17-495.
Althoucgh both of these regions bound about equally well to the RNA polymcrasc core
cnzymc. il] \-itro transcription assays revealed that the amino-terminal RNA polymerase-
binding donlain of NusA is very important for NusA's effects on termination at an intrinsic
tcrminritor and on antiterminrition by N. while the carboxy-terminal RNA polynierase-
binding rcgion is dispensable for both activities.
In addition to the prcviously obscrved interaction bctwcen N and full lcngth NusA
(Greenblatt and Li. 198 I b). N protein also bound NusA ( 1-416). However. Surthcr truncation
of NUSA. to amino acid 318. resulted in the loss of N-binding. There are two xidic repeats in
the carho?-terminal region of NusA (Craven et (11.. 1993). One of these rcpczits is Iost when
YLISA is tnincated to amino acid 416, while the second is Iost when NusA is Sunher tnincated
minimal functional protein
RNAP binding RNA interactions terminalion terminalion antitermination antitermination --
RNAP binding Binding to
RNAP Term.
N-nu& Binding N-NusA-nut bhuMh~sA- NusA,B,E,G
ta N cornplex RNAP cornplex complex Antlterm.
Figure 9: Siiiiiiiiiiiy of ilic Rc~iilis «t' itic Deleiioii Aiiiilysis »f NUSA. Scc icxt for dciiiils.
to amino acid 348. it may be that the basic N protein interacts with both of these regions
equally weH and that loss of one of the two acidic repeats has only a minor effect on N-
bindins.
Results from the in i i t ro antitermination assay, where loss of N-binding ability did
not affect NusA function. calls into question the importance of the N-NusA interaction.
Howmw-. 1UI1 length NusA cannot interact with RNA on its own. and I speculate that the
binding of N to the carboxy-terminal region of NusA facilitates the binding of NusA to RNA
in the contcst of the antitermination complex. indeed. interaction of NusA with RNA
appears to hc essential for antitermination because the nrrsA/ point mutation in the S 1
honiolog>r rcgion of NusA (NusA R183A: Friedman, 197 1 ) or deletion of the S 1 and KH
honiolog>~ r q i o n s in NusA ( 1 - 137) each can impair NusA's ability to support antitermination
hy N. Thlis. I think that N may transforrn wild type NusA from a termination factor into an
antitermination factor by promoting a NusA-nrtr site RNA interaction that woulci lead to
rtntiterniiniition rather than an aiternative NusA-RNA interaction that would Icad to
rumination or pausing.
Forinrition of complexes in gel mobility shift experiments gives an indication of thc
stabilit~r O f a corn plex. E3y using the NusA deletion constructs with this assay. i~ was possiblc
t« dctcrniinc what regions of NusA are important for interaction wi th the othcr components
of thc systcrii. When gel shifts were performed with N. NusA and the irrtr sitc. loss of the N-
binding rcgion or the S 1 homology region prevented the formation of the N-NUSA-mit site
coniplcs. h ~ i t neither RNA polymcrase-binding region was essential for the forniation of this
complex. When the reactions contained RNA polymerase. as well as N. NusA. and rut/ site
RNA. thc 3-binding region and one or the other of the RNA polymerase-binding regions of
NusA Lvcrc critical for the torrnation of the characteristically unstable RNA polymerase-
containing complex that firnctions in antitermination only over a short distancc (see Figs. 8
and 9: Whalen er al.. 1988: DeVito and Das. 1994; Mogridge el al.. 1995: Rees e t al.. 1996).
The abiliiy of NusA (132-495) to f o m this complex. as well as a more stable complex in the
presence of NusB, NusG. and S 10, even though this complex lacks the amino-terminal RNA
polymerase-binding region of NusA and is non-functional in antitermination. irnplies that N
riltrrs the iiniino-terminal termination-enhancing region of NUSA so that it üctiiülly interferes
with terminrition by RNA polymerase.
SUSA ( 1-338) was unable to support the formation of observable arnounrs of the N-
SusA-RSA polymerase-nlrr site complex in a gel mobility shift assay. Howe\.er. as more
Sus factor\ Ivere added to the reaction. so that there were additional protein-protrin and
protein-RNA interactions. NusA (1-348) was able to support the formation of a Ionr mobility.
htablc coniplex. Thus. NusB. NusG. and S 10 can restore the stability of the coniplex and
compensate t'or loss of the carboxy-terminal RNA polymerase-binding region and N-binding
rcgion in NcisA ( 1-338). Interestingly. although the carboxy-terminal RNA polymerase-
binding rcgion could not compensate for the loss of the amino-terminal RNA polymerase-
binding region in transcription assays. it could do so in the gel mobility shifi esperiment.
suggcsting that. whereas cither protein-protein contact betnveen NusA and RXA polymerrise
is cnoiigh to stabilize the complex. only the amino-terminal interaction has very criticai
conscqiicnccs for termination and antitermination. This is consistent with the observation
that an E.c.o/i strain containing NusA ( 1-3-43). which lacks the carboxy-ternlinal RNA
pol~*mcrasc-binding region. is viable. although temperaturc-sensitive. and supports partial
antiterminiition by N (Tsugawa el tzl.. 1988). Whereas NusA ( 1-338) supports both
termination and antitermination. neither NusA ( 1-137). which binds RNA polynierase. noi-
XusA ( 132-348). which contains the S 1 and KH homology resions. can function on its own.
t.\.cn thoiigh CD and NMR analysis indicate that these NusA fragments have highiy ordered.
tolded structures. 1 have not determined whether a combination of NusA ( I - 137) and NusA
( 132-348 ) is functional.
The S 1 and KH homology regions are predicted RNA-binding domains (Gibson C r
CI/.. 1993; Bycroft et al.. 1997). Gel mobility shift experiments with r ~ l i t site RNA have
shown that point mutations in the S 1 homologg region are detrimental to the formation of
various RNA-bound complexes containing N just as they are detrimental for antitermination
(Chapter 4: .Mogridge et cil.. 1995: Y . Zhou and D. Friedman. unpublished data). My
observations here that NusA (232-416) and NusA (232495). two constructs w.hich lack the
S 1 homology region and are likely to be folded. do not bind hi-nrtr site complescs. d s o
sugsest thrit the S 1 homology region is important for complex formation. Although NusA
( 1-240 1. n-hich lacks the KH homology regions, did not support termination. antitermination.
or thc torrnation of RNA-bound complexes. CD and NMR experiments indicatcd that the S 1
hornolog~. rcgion in this construct is unlikely to be folded. Therefore. it is still unclear
~vhethcr the KH homolo~y regions of NusA participate in tcrmination. antitcrmination. o r
RNA-binding.
J . Mosridge and 1 have shown that there are nucleotides in both box4 aiid hoxB that
are important for NusA association with the N-nlrt site complex (Chapter 4: Mogridge er trl..
1995). I t lias also been shown previously that ribosomal protein S 1 specifically binds i70 .rA
RNA t .Mogridge and Greenblatt. 1998). suggesting that the S 1 homology region of NusA
may intcriict with ho-uA. if the S 1 homology region interacts with one portion of the nrrr sitc.
i t is possihlc that at leait one of the KH hornology regions interacts with the other. As there
is evidencc that individual KH domains have different sequence specificities (Dejgaard and
Lcffcrs. 1996). it is possible that one of the KH homology regions in NusA is required for nrtt
si tc bindin: and the other is required to bind to nascent mRNA. An alternati\-c possibility is
providcd bj. the observation that the hnRNP K protein has three KH domains and mutation or
dcletion of' any one of then1 severely affects the ability of the protein to interxt with C-rich
scqucnct.?; t Siomi et cil.. 1994). This suggests that both KH homology regions in NusA may
bc rcqiiircct for binding one type of RNA sequence.
The amino-terminal RNA polymerase-binding domain and the S 1 and KH homology
regions of NusA are the minimal regions required for wild-type function in Our in i-irro
assays for tcrmination and antiterrnination. Further work needs to be donc to ciucidate the
RNA seq ucnce preferences of the S 1 and KH RNA-binding regions in NusA and to identify
the precise tùnctions of the critical interaction between the amino-terminal domain of NusA
rind RNA pol ymerase.
Experimental procedures
Plasrnids. enzymes, and strains
RNA polymerase. N. NusA. NusB. and NusG were purified as previously described
i Burgess and Jendnsak. 1975; Greenblatt et al.. 1980; Greenblatt et al.. 198 1 : Swindle et tri..
1988: Li cPr (11.. 1992). Puri tied S 10 was generously provided by Dr. Volker Nowotny.
Al1 plasmicfs were prepared in the bacterial strain DH5a (Life Technologies. Inc.) and
puri tkd on Qiagen-tip 500 columns (Qiagen). The ~Iigonucleotides used for cloning werc
purc hasecf from ACGT Corp. (Toronto). RNAguard was bought from Pharmacia Biotec h.
Restriction cnzymes and DNA ligase were purchased from Kew England Biolabs. T7 RNA
polymc~isc was obtained from Life Technologies, Inc.
Construction of His,-tagged NusA Proteins
PCR primers were designed to arnplify NusA or fragments of NmA frroni the plasmid
pJL1. For\vxd and reverse primers contained Ne01 and BtauHI restriction sites. respectively.
for subscqiicnt cloning into the vector. PET-1 Id (Novagen ). The foward primcrs also
contriincd thc sequence for six histidines.
Purification of His,-tagged NusA Proteins
Thc Ecoli strain BLZI (pREP,j containing the His,-tagged NusA plasri-iids was
mown in I I of LB medium to an A, of 0.5 and induced for 3 hours with 3 niiM isopropyl- 1 - C
t hio-P-D-@ctopyranosidc. Cells were harvested by centrifugation for 20 minutes at 4500
rpni in a Son-al1 H6000A swinging bucket rotor. resuspended in 10 mL binding buffer (5
mM imidazole. 0.5 M NaCl. 20 mM Tris [pH 7-91, 10% glycerol. 5 mM P-mercaptoethanol).
sonicated. and centrifuged for 30 minutes at 15 000 rprn in a Sorvall SS34 rotor. The
cxtracts were mixed with 0.25 mL His Bind beads (Qiagenj and incubated at 1°C for 2 hours.
Tlie beads nxre added to 1 ml Bio-Spin columns (BioRad) and washed sequentially wiih 1 ml
binding buffcr. 1 ml wash buffer (45 mM imidazole. 0.5 M NaCl. 20 mM Tris. [pH 7-91. 10%
glycerol. 5 miM P-rnercaptoethanol ) and 1 ml ACB ( 10 mM HEPES [pH 7.01. 10% glycerol.
0.1 m M EDTA. 5 mi! P-mercaptoethanol conraining O. I M NaCI). Columns nxre eluied
with 1 ml ACB containing 0.1 M NaCI. 0.4 M irnidazole [pH 7-91, and 1 mM DTT.
tlffini ty Chromatography
For csperiments with GST-N. extract containing GST or GST-N (Mogridge rr ai..
I998a n~ incubated with Glutathione Sepharose 4B beads ( Pharmacia Biotcch i and rotrited
ar rooni ttsnipcrature for 15 min. The beads were wcished sequentially with lkl NaCl B u f i r
A (20 n1.M Tris-HCI [pH 7.51.0.2 rnM EDTA, 1 mM Dm. 1 mM PMSFi and 0. I hl NaCl
Bufkr A. B u d s (20 PL) were added to 200 pl pipette tips that contained 10 pl of 212-300
micron glass beads (Sigma). The column bed was washed with 10 colurnn volcimes of I LM
NaCl ACB ( IO miM HEPES [pH 7-01. 10% glycerol, 0.1 m.M EDTA. 1 mM DTT) and thcn
washed \\sith 10 column volumes of 100 m M NaCI ACB. Columns were loiidcd with 100 PL
of a misrurc of His,-taggcd NusA and His,-iagged NusA dcletion mutants (3.0 pg each) in
100 riiM XlC1 ACB buffer supplemented with 0.2 rng/ml insulin. The col~in~ns wcre washcd
tvith 1 O colir~nn volumes of 100 mM NaCl ACB and eluted with 1 M NaC1 ACB. One
quartcr 01' tlic eIuate was loaded in the gel.
For cspcrinicnts with RNA polyrnerase and the His,-tagged NusA and the Hi';,,-ragged NusA
dcletion riiritants. purifieci His,-tagged protein was incubated with Ni-agarosc (Qiagen) for 30
min. 20 ul ol' beads were added to 200 pl pipette tips and treated as descnbed abovc except
that these columns were loaded with 100 pL of 1 mg/mL E-coli extract containing 6 pg
additional RNA polymerase core enzyme.
In vitro Transcription
Tr;inscription reactions were perforrned as previously described (Whrtlen and Das.
1990) using the template pJD 12 (generous gift of A. Das). Concentrations of proteins are
indicnted in the figure legends. Levels of terminated and run-off transcripts were quantitated
using a phosphoimager.
Gel Mohility Shift Experiments
Ge1 mobility shiti assrtys were perforrned as previously described (Mogridge et a l . .
1995 ). Concentrations of proteins are indicated in the figure legends.
CD and NMR Spectroscopy
Circular dichroism was perforrned in an Aviv 62A DS circular dichroisrn
spcctromctcr. Ali measurements were collected at 25°C in 10 mM Tris-HCI, O. I mM EDTA.
250 n1.34 SaCl and 0.1 mM DTT in a O. 1 cm cuvette. Spectra were collected at a scanning
speed of 1 nndsec from 300 nm to 200 nm. "N-labeled fragments of NusA were prepared
and HSQC NiMR spectra wcre generated as described previously for the 7~ N protcin
t Mogridgc ~~r cl!., 1998).
The a Subunit of E.coli RNA Polvmerase Activates
RNA-Bindine bv NusA
Abstract
SusA modulates pausing, termination. and antitermination by associatin_o with the
Ecnli R S A polymerase corr enzyme. Although NusA cross-links to nascent RNA within a
transcription cornplex, it does not associate directiy with RNA on its own. 1 have shown hcre
that both the carboxy-terniinal domain of the a subunit of RYA polymerase and the
bacteriophagr h N gene antiterminator protein bind to carboxy-terminal regions of NusA.
Partial deletion of these same regions allowed NusA to bind RNA on its own. Interaction
with u t or the k N protein) resulted in RNA-binding that involves the S 1 honiology re, ion of
NusA. Thc interaction of u with NusA may enable NusA to bind the nascent transcript and
thus sti mulatc pausing and termination. The N protein may reverse the efkcts of NusA on
pausing and termination by inducing NusA to interact with N-utilization ( r z r r t ) site RNA
ratfier than RNA near the 3' end of the nascent transcript.
Introduction
The NusA protein of E-coli binds to core RNA polymerase shortly atier the initiation
of transcription and stimulates pausing and terrnination rit certain sites (revietved in
Richardson and Greenblatt. 1996). The mechanism by which NusA influences pausing
during transcription is not yet clear but RNase protection experiments suggest that NusA may
bind and stabilize the stem-loop RNA structures often associated with pause hites (Landick
and Yanokky. 1987).
NrisX binds directly to the a subunit of RNA polymerase (Liu et cil. , 1996). [t can
a!so be cross-linked to the large P and P' subunits of RNA polymerase (J. Li and J.
Grccnblritt. ~inpublished data) and may be capable of binding directly to these subunits as
well ( Liu L'I (11.. 1996). The a subunit o f RNA polymerase ha two domains: the amino-
terminal doniain (NTD) is required for dimerization and for interaction with the P and P'
siibiinits 01' RNA polymerase. whereas the carboxy-terminal domain (CTD) is a contact
surface h r - DNA-binding activator proteins (reviewed in Ebright and Busby. 1995). The
CTD 01' u also binds the UP element. a DNA element that enhances initiation of transcription
;it ccrtain promoters (Ross et (il . . 1993; Blatter et cil., 1994). Additionülly. it hris been
wggt'sred that a direct interaction between the a-CTD and NusA is important for NusA's
ability to control pausing and termination (Liu et cil.. 1996).
XcisA influences not only pausing and terrnination by RNA polymerastl but also
tr;inscriptional antitermination by the bacteriophage k N protein (Friedman. 197 1 ).
Antitcrniination by N requires a cis-acting RNA element, called the r u r r site. which consists
of two functional components. bo-rA and hoxi3 (Salstrom and Szybalski. 1978: de
Crombrrigghe et cd. . 1979: Olson et al.. 1982; Das and Wolska. 1984: Horwitz et cd.. 1987).
NusA. as wcll as NusB. NusE (ribosomal protein S IO), NusG. RNA poiymerrisc and the r iu t
site RNA. tlike part in multiple interactions within the N-modified transcription complex
(retrieured in Friedman. 1988: Greenblatt et al., 1993). The resulting highly stable
ribonuclcoprotein complex is capable of suppressing transcription termination over long
distances and through multiple terminators (Mason et al.. 1992; Mogridge et d.. 1995).
Within this complex, NusA interacts with the N protein (Greenblatt and Li. 198 I b). and both
the arnino- and carboxy-terminal regions of NusA interact with RNA polymerase (Chapter
2).
Thcrc is evidence that NusA may interact directly with nucleotides in both the 60xA
and /70-t-B components of the nrrr site (Olson et al., 1982: Friedman and Olson. 1983; Oison et
cd.. 1984: .Mogridge et cd . . 1995)- The effects of mutations in the S 1 homology region of
NusA bcttvcen amino acids 136 and 240 suggest that this region is important for
antitermination. The ncrsA / (L l83R) and rriisA R199A mutations both cause temperature-
sensitive i, growth because of an inability of N to function at high tempenturc (Y. Zhou and
D. Friedrnm. unpublished data: Friedman. 197 1 : Friedman and Baron. 1974). Cnlike wild
type XusA. both mutant proteins are unable to supershift an N-nrtt site RNA cornplex in a gel
mobility shili experiment. cven though they bind N directly with wild-type rtffinity (Chapter
1; T. blah and J . Greenblatt. unpublished results; Mogridge et cd.. 1995). This suggests that
both n-iutlitions cause a defect in the interaction of the S 1 homology region of NusA with tllrt
site RKA. Other experiments have shown that both an amino-terminal RNA polymerase-
binding rcgion in amino acids 1- 137 and a portion of NusA ttiat contains the S I and KH
homology rcgions are essential for NusA to enhance both termination at an intrinsic
terminritor and antitermination by N (Chapter 2).
Dcspite this abundant evidence suggesting that NusA can interact with RNA. it does
not rissociatc directly with RNA in a gel retardation assay (~Mogridge er cd.. 1995). Rather. its
ability to interact with phase nut site RNA depends on the presence of N (Mogridge et al..
1995). ti-tiich interacts dircctly with NusA (Greenblatt and Li 1981). Since NusA does cross-
link to RXA in a transcription complex in the absence of N (Liu and Hanna. 19951, it seemed
likely that an interaction of NusA with RN.4 polyrnerase might also alter the conformation of
NusA so as to allow for RNA-binding. 1 report here that interaction of the RNA polymerase
cx subunit with NusA promotes the interaction of NusA with RNA. My observation that
a and the À N protein bind to sirnilx regions near the carboxy-terminus of Nus.4 suggests
that a and N may act in similar ways to control the binding of NusA to RNA. My results
suggest that an interaction of NusA with a in a transcription complex would allow NusA to
bind the nascent transcript and stimulate pausing and termination by RNA polymerase. The
À N protein may reverse the effects of NusA on pausing and termination by causing NusA to
interact Lvith rlut site RNA rather than the RNA near the 3' end of the nascent transcript.
Results
Bindinr o t- NusA to RNA in the presence of the RNA polvmerase a subunit
In vicw of the known involvement of the a subunit of RNA polymerasc in NusA
titnction (Lit1 et ni.. 1996). I tested whether a could promote RNA-binding by NusA in gel
mobilit?. shift assays containing "P-labeled RNA with a wild type kmtt site tFig. 1). The
addition of increasing amounts of a to a constant amount of NusA resulted in thc appearance
01' tneo bands. and sometirnes a weak third band, with lower rnobility than the free RNA (Fig.
l a. cornparc lane 1 with lanes 7- 10). These bands were absent in lanes containing either
NusA donc (Fig. la. lane 2 ) or a alone (Fig. la, lanes 3-6). indicatint that formation of a
cornplcs on the RNA required both proteins. In the converse expriment. thc concentration
of u. \va> hcld constant and that of NusA was varied (Fig. I b). In this case. XusA done was
not ahle to hind rrrtt site-containing RNA even at very high concentrations (Fig. 1 b. lanes 2-
4). but coniplexes of lowcr rnobility appeared and increased in intensity as the concentration
A NusA NusA
Figure 1: Binding of NusA to RNA in the presence of the RNA polymerase a subunit. (a) Addition of increasing amounts of a to a constant amount of NusA results in an increase in complex formation. Reactions containing 32~-labelled nul site RNA and various combinations of 14 pM NusA and 1.25, 2.5, 5, or 10 FM a (as indicated) were electrophoresed on 7.5% non- denaturing gels, dried, and exposed to film. (b) Addition of increasing amounts of NusA to a constant amount of a results in increased complex formation. Reactions containing 32~-labelled nut site RNA and various combinations of 9 FM a and 3.5, 7, or 14 FM NusA (as indicated) were electrophoresed on 7.5% non-denaturing gels, dried, and exposed to film.
of NusA was increased in the presence of a (Fig. Ib. lanes 6-8). In view of the known ability
of a to dimerize p n m ~ l y via its arnino-terminal domain (Blatter er al.. 1994: Kimura et cr f . .
1994). thcse complexes may represent different combinations of a and NusA.
The CTD of t h e RNA ~~(vrnerase a subunit stimulates RNA-bindin~ bv NusA
Because the CTD of the RNA pofymerase a subunit is known to be important for
NusA actiieity in pausing and termination (Liu et rrl., 1996). I tested whether the a-CTD
(amino acids 249-329) or oc-NTD (amino acids 1-235) alone could promote RNA-binding by
NusA (Fig. 2). 1 added full length a. a-CTD or a-NTD to gel mobility shifi rcxtions with
NusA and "P-labeled n i t r site-containing RNA. Neither the a subunit nor its isolated amino-
terminal ~ind carboxy-terminal domains alone coutd bind to the RNA (Fig. 2. Imes 3-6). By
con trast. a carboxy-terminal1 y truncated NusA derivative containing amino acids 1 -4 16 is
capablc 01' direct binding to r l t t t site-containing RNA (Fig. 2. lane 2: also see bclow). The
addition ot-cither intact a (Fig. 2 , Ianes 7 and 8) or the a-CTD (Fig. 2. lanes 9 and f0) to
intact NLKA (amino acids 1-495) caused shifts in the mobility of the RNA. Thc niobility of
the cornples formed with the a-CTD and NusA (Fig 2, lanes 9 and 10) was similar to that of
thc more rapidly migrating complex obtained when full length a utas incubatcd with NusA
(Fig. 2. liincs 7 and 8). Since the a-CTD lacks the principle dimerization doniain of a
(Blattcr C I cd.. 1994: Kimura ur al.. 1994). the complex obtained with NusA and the a-CTD
likcly contains only one molecule each of the a-CTD and NusA. The additional lower
n-iobility coniplcx in the rerictions containing a and NusA (Fig. 2. lanes 7 and Y ) likely
contains tivo moIecules of a and one or more molecules of NusA as a conseqiicnce of
ciimcriz~iiion of the a subunits. In contrast. no distinct shift was obtained with NusA and the
Figure 2: a-CTD stimulates RNA-binding by NusA. Reactions containing 32P- labclled u i ï r site RNA and various combinations of 13 pM NusA ( 1 3 16). 4.5 or 9 p M or. 1.5 or 9 p M aCTD or 1 1 p M aNTD (as indicated) were electrophoresed on 7.5% non-denaturing gels, dried, and exposed to film.
a-NTD (Fig. 2. lane 1 1 ). These results indicate that the CTD of the RNA polymsrase a
subunit is capable of s t i m u l a h g RNA-binding by NusA.
RNA-bindine Sv NusA in the presence of a is seauence-specific and sensitive io a mutation
in the S 1 homoloov region o f NusA
To ewlua tc potential RNA sequence o r structure specificity in the RNA-binding observed
with NusA and a. 1 compared the abilities o f RNAs containing either a wild tjvpc nitr site or a
r r u r site in \\.hich the sequence of the box4 element had k e n switched from 5--3* to 3.-5'
(hu.i-i\ rw.crsc) (Mogridge C I ~11.. 1995) to support formation of NusA-oc-nitr site complexes
(Fig.3a). Whereas N only requires the bosB RNA element for binding, NusA is unable to
supershit't Lin Y-rzitr site complex when bosA is reversed (Mogndge et cd.. 1995 1. This
sugge'its that there is a direct and specifïc interaction between bosA and an RNA-binding
domain in NiisA. As shown in Fig. 3a. the low mobility complexes formed n i t h the wild
type prohc. SusA. and u were not present in reactions when the reverse riut site probe was
used ( Fig. 3a. compare lane 4 with lane 8). Thus. the NusA-binding promotcd by a in thcsc
rcl niobi 1 it', shi ft experiments has structure o r sequence-specificity. 1Moreovèr. the C
importance of the bosA element for ki t \ -binding provided additionai evidencc chat the
RNA-binciins observed in cxperiments containing NusA and or involves NusA.
The rirrsA R I 99A mutation causes a defect in antiterrnination by N in \iiqo. as ~ve l l as a defect
in the ribility of NusA to supershift an N-niit site complex. even though the NusA R199A
mutant protcin binds with normal affinity to N (Chapter 4: T. iMüh. Y. Zhou. J. Greenblalt
and D. Friedman. unpublished data). Therefore, this mutation in the S 1 homology region of
YiisA appcrrrs to cause a defect in the binding of NusA to ui r r site RNA. Thc l~lrsA Rf 99A
mutation also prcventcd NusA from binding the nur site RNA in the presence ot'a (Fig. 3b.
comparc Iancs 6 and 7). This result indicates that the S 1 homology region of NusA
is likely to participate in nitr site-binding stimulated by CI and provides further cvidence that
RNA-binding observed in the presence of a and NusA reflects direct RNA-binding by NusA.
Interaction of a with the cûrboxv-terminal region of NusA
My observation that NusA required a to bind nrtt site RNA suggested that there may
be a direct interaction between NusA and a. To test for a direct interaction between a and
NusA. a mixture of full-length NusA and three carboxy-terminally deleted mutant proteins
(Fig. -Ca. lane 2 ) was passed over columns containing various concentrations of covalently
bound a. Specific binding to a was observed only for the full-length NusA protein. whose
binding incrcased in concert with the a concentration on the column (Fig. .Ca. lanes 4-6).
Since NtisA ( 1-416) did not bind to a. the carboxy-terminal 79 amino acids of NusA are
necessu-y tor the binding of a to NusA. To further establish which regions of NusA are
suffïcient tbr interaction with a. purïfied a was tested for binding to various covalently-
immobilizcd portions of NusA: an amino-terminal region. NusA ( 1- 137). that I have shown
binds R X A polymerase (Chapter 2); a carboxy-terminal fragment. NusA (303-495); and the
tù11 lengrh protein (Fig. 4b). a did not bind to the arnino-terminal fragment ot' NusA (Fis.
-Sb. Ianc 4). Appreciable binding to the carboxy-terminal rcgion of NusA was obsenred (Fig.
Ib . Ianc 5 1. and the binding of a was best with full length NusA (Fig. 4b. lanc 61, It appears
that a intcrricts pnmanly with the carboxy-terminal region of NusA. aithoush the amino-
terminal (\\.O-thirds of NusA may also contribute to the binding.
Since the a-CTD but not the a-NTD allowed RNA-binding by NusA (Fig.2). 1 also
tcsted uvhiçh region of a was involved in the direct binding of to NusA ( F i s 4c). Fuli
lcngth (y.. ci-NTD and a-CTD were mixed together (Fig. 4c. lane 2) and loadcd ont0 columns
contüining GST (Fig. 4c. iane 3) or various concentrations of GST-NusA (303495) (Fig. 4c.
C I I r J A d a ~ E E E E o a a a a
E E E E
NusA Figure 4: Interaction of a with the carboxy-terminal region of NusA. (a) Carboxy-terminal turncation of NusA prevents the a-NusA interaction. A mixture of four Hisg-tagged NusA proteins (lane 2) was passed
over columns containing aff igel (lane 3) or increasing amounts of aff igel-coupled a (lanes 4-6). (b) The carboxy-terminal region of NusA interacts directly with n. Buffer containing a and 0.2 mglml insulin (lane 2) was passed over columns containing affigel (lane 3) or affigel-coupled NusA (2 mglml) (lane 6) or affigel- coupled regions of NusA (lanes 4 and 5) (as indicated). The concentrations of amino- and carboxy-terminal regions of NusA on the columns were adjusted so that each had the same molar concentration as the full length NusA. (c) CYCTD interacts with the carboxy-terminal 192 amino acids of NusA. A mixture of a, aCTD,
and aNTD (lane 2) was passed over columns containing GST (lane 3) or increasing amounts of GST- NusA (303-495) (lanes 4-6). As a control, buffer alone was passed over a GST-NusA (303-495) column (lane 7). Bound proteins were eluted with buffer containing 1 M NaCI, subjected to SDS-PAGE, and stained with silver. ' indicates a degredation product of a, as identified by mass spectrometry.
lanes 4-61. The bound proteins were eluted with sait. None of the a fragments were retained
on the controI GST column. In contrast. as the concentration of imrnobilized GST-NusA
(303-495) on the coiumns was increased. increasing arnounts of full length a and a -CTD
Lvere prcscnt in the sait eluates from this matrix. The binding of a-NTD to the imrnobilized
GST-NLI~A (303-4951 wfas barely detectable (Fig. 4c. lanes 4-6). Therefore. the result of this
direct protein-protein binding study was consistent with the resuits of the gel niobility shifi
espçrirnents. which also indicated that only full-length a and the a-CTD wouid interact with
NusA.
A carhosv-terrninallv truncated NusA also binds specifically to rzrrr site RNA
>f'. observations that a could provoke RNA-bindins by NusA and interact with NusA
( 1-495 1. but not with Nus.4 ( 1 - 4 16) suggested that the carboxy-terminai 79 amino acids of
SusA niight inhibit the RNA-binding activity of NusA. Lhlike full-length NusA. NusA ( 1 -
11 6) could bind RNA containing a wild type niif site in a geI mobility shift experiment (Fig.
,Sa. Iancs 2-7). but bound only weakly to RNA containing a rz t rr site with a rtxwsed boxA
sequcncc (Fig. 5a. lanes 12- 14). This result indicates that the binding of NusA to luit site
R N A is indced inhibited by the carboxy-terminal 79 amino ricids of NusA and suggests that
this inhibition could be relicved by an interaction of the carboxy-terminal rcgion of NusA
with thc C T D of the RNA polymerase a subunit,
In order to further characterize the a-independent RNA-binding by Nris.4 ( 13 16).
other deletion mutants of NusA were tested for their ability to bind RNA containing a wild
type I I M ~ site in a geI mobility shift experiment (Fig. 5b). Truncation of NusA to amino acid
318 u-ciikencd RNA-binding. even though NusA ( 1-348) retains the S 1 and KH hornology
rq ions ( Fis. 5b. lane 4). The deletion of NusA's amino-terminal RNA polynierase-binding
rcgion, as in NusA ( 132-1 16). prevented RNA binding by NusA (Fig. 5b. lane 1 ). Therefore.
Ad 44 NusA(132-416) - + - - - - wt NUSA NusA 1-416 wt NuaA NUSA 1-416 NusA(132-495) - - + - - -
NUSA (1-348) - - - + - - wt nut site reverse nut site
NusA(I-416) - - - - + - NusA (1-495) - - - - - +
Figure 5: A carboxy-terminally truncated NusA binds specifically to nut site RNA (a) RNA-binding by NusA (1 -416) is prevented by a mutation in the boxA portion of the nut site. Reactions containing wild type or mutant 32~-labelled nul site RNA (as indicated) and 3.5, 7, or 14 FM NusA or NusA (1 -41 6) (as indicated) were electrophoresed on 7.5% non-denaturing gels, dried, and exposed to film. (b) RNA-binding by NusA (1-416) is stabilized by amino acids 1-137 and 348-41 5 of NUSA. Reactions containing 32~-labelled nut site RNA and 10 FM NusA (1 32-416), NusA (132-495). NusA (1 -348). NusA (1-416) or NUSA (as indicated) were electrophoresed on 7.5% non-denaturing gels, dried, and exposed to film.
the amino-terminal RNA polymerase-binding region in amino acids 1- 137 of NusA, as well
as amino acids 348 to 415 of NusA. appear to be required to stabilize the RNA-binding
ability of XusA ( 1-416).
The  N protein also binds the carboxv-terminal region of NusA
The N protein of the bacteriophage M s o binds NusA (Greenblatt and Li. 198 1 b) and
1 have shown that this binding requires a carboxy-terminal region of NusA tchrtpter 2).
Ivloreovcr. SusA (1-348). which lacks this region. cannot supershift an N-nrrl site complcx
(Chüpter 2). In view of the possibility that a and N might act in similar ways to provoke
RNA-binding by NusA. 1 tested various fragments of NusA for binding to N. Full length
Ornent N u s A a carboxy-terminally tmncated NusA, NusA ( 1-399). and a carboxy-terminal fra,
of NiisA. XtisA (303395). were mixed with E-coli extracts and passed over GST and GST-N
riftinity coltimns (Fit. 6) . None of the major E-coli proteins present in the extracts applied to
the columns bound to the GST or GST-N columns (Fig. 6. lanes 2.3. 5.6. 8 and 9). Both
XusA and SusA (303495) bound selectively to GST-N and therefore were prcscnt in the
high sd t cltiates from the GST-N columns. but not in GST control column eluatcs (Fig. 6.
lancs 2. 3. (Y. and 9). NusA ( 1-399) was absent from the high salt eluates of both columns
(Fis. 6. liincs 5 and 6). Thus. N binds dircctly to the carboxy-terminal region of NusA. This
rcsult suggcsted that N might activate the RNA-binding ability of NusA by binding to the
wmc rcgion of NusA that 1 have shown binds a.
- NusA
+ NusAl-399
- -
NUSA NusAl -399 NusA303-495
Figure 6: The h. N protein binds directly to the carboxy-terminal 192 amino acids of NUSA. Ecoli extract containing additional NusA (lane 1 ), NusA ( 1-399) (lane 4) or NusA (303-495) (lane 7) was passed over columns containing 2 mg/ml GST (lanes 2 . 5 . and 8) or 0.5 mgml GST-N (lanes 3.6, and 9). Bound proteins werc cluted ~ . i t h buffer containing IM NaCl. subjected to SDS-PAGE, and stained with silver.
Discussion
An Autoinhi bition Domain Mav Inhibi t RNA-binding by NusA
Whereas fiitl length NusA does not bind RNA, 1 have shown here that a carboxy-
terminal deletion mutant, NusA (1-416), which retains the S 1 and KH hornology regions of
XusA. c m bind RNA in the absence of a or N. This suggests that one or more of the RNA-
binding domains of NusA may be occluded by the carboxy-terminal79 amino acids cf NusA
(see Fig. 7a) . Since neither NusA ( 1-348) nor NusA ( 132-4 16) binds RNA as strongly as
NusA ( 1-1 16). arnino acids 1 - 13 I and 3 4 9 3 1 6 must also alter the folding. alignment. or
accessibility of the RNA-binding domains. Interaction of the carboxy-terminally tmncated
NusA ( 1 - 4 16) with RNA is sensitive to alteration of the hc~v4 portion of the t i r r t site (Fit . 5a).
This is also the case when full length NusA binds the m t r site RNA in the presence of N
( iblogridgc ci crl.. 1995) or a (Fig. 3a). RNA-binding by NusA ( 1 - 4 16) is also inhibited when
alteraiions in the loop of ho.i.B prevent Doxi3 from forrning a GNRA tetraloop-like structure
(ses Chaptcr 4; Legault et (11.. 1998). It is unclear. however. which domains of NusA ( 14 16)
contributc to the sequence- or structure-specific interactions of NusA with Irlrr site RNA.
The inability of full-length NusA to bind RNA resembles the inability of thc intact initiation
subunii O-" of RNA polymcrase to bind DNA unless it is part of RNA polymerase
holocnzyrnc (Dombroski et c d . , 1992; Dombroski el ni., 1993). Just as deletion of the
carboxy-terminal region of NusA enables NusA to bind RNA in the absencc of' RNA
polynlcrasc. deletion of the amino-terminal 130 arnino acids of 0" enables O-" ro interact
~peci!?c:ill~. and non-specitkalty with DNA in the absence of the other RNA polymerase
subuniis (Dombroski et cil . . 1992). The autoinhibition of DNA-binding by 0'" iiiay be
rclievcd u*hcn 0" interacts with the core polyrnerase subunits and under, es a
contorniational change that uncovers or reorients its DNA-binding domains (Dombroski et
RNA
NusA alone
pause or terminator RNA nut site RNA
Figure 7: Mode1 for NusA function in elongation, termination and antitermination. See text for details.
d.. 1997: Dombroski el al.. 1993: Malhotra et al., 1996). It is intriguing that tw.0 prokaryotic
proteins that c o m p t e for binding to the RNA polymerase core enzyme (Greenblatt and Li.
198 1 a ) may use similar mcchanisms to control their nucleic acid binding ability.
Interaction o f NusA with cx : Relationship to RNA-bindina, and Transcription Termination
51:. observation that the a subunit of RNA polymerase stimutates RNA-binding by
Nus.4 i h consistent with previous studies showing that NusA c m be cross-linked to the
nasccnt transcript in transcription complexes (Liu and Hanna. 1995). 1 propose that this
effect of u. on RNA-binding by NusA is the consequence of a direct interaction between the
two protcins (see Fig. 7b). The bindinp of NusA to a and stimulation of RNA-binding by a
rire n~cdirited by the carboxy-terminal domain of a (Figs, 2 and Jc)- The a-CTD was
implicritcd previously in the ability of NusA to stimulate pausing and termination by RNA
polyrnerasc (Liu et cd,. 1996). RNA-binding either by truncated NusA or by full length
NusA in thc presence of oc is weak. Nevenheless. the dependence of RNA-binding by
truncated NusA or by full lcngth NusA in the presence of a on the boxA sequcnce in the tzur
site is siniilm to the effect of box4 on RNA-binding by NusA in the presence of N (Mogridge
et tri.. 1995 1. .My ability to abrogate RNA-binding by deleting the a-CTD or portions of
NusA rilso siiggests that RNA-binding genuinely involves both a and NusA.
From the data presented here. 1 hypothesize that full-length NusA is prcvented from
interacting \vith RNA by an autoinhibition mediated by its carboxy-terminal 79 amino acids
(compare Fis. 7a with Fig. 7b). I suggest that during elongation. NusA uses its RNA
polyrnerasc-binding region in amino acids 1 - 137 (Chapter 2) to interact with R N A
pol y nicriisc subunits P and P'. and its carboxy-terminal region to interact wit h u (Fig. 7c).
The interaction with a may then cause a conformational change in NusA such that its RNA-
binding domains either fold or become exposed and competent to bind the nasccnt RNA.
Thus. as part of the transcription cornplex. NusA would bc in a position to bind and stabilize
pause and tcrrnination motifs in the nascent RNA. leading to enhancement of pausing and
termination at certain sites. 1 suggest that the interaction of the a-CTD with NusA is
essential for NusA to stimulate termination only if the inhibitory carboxy-terminal region of
NusA is prclsent and not if it is deleted. In otherexperiments 1 have shown that NusA (1 -
4 16) is riblc to stimulate termination (Chapter 2). This fraement of NusA cannot interact
with a. but i t lacks the inhibitory region and can bind RNA on its own.
The interaction between the amino-terminal RNA polymerase-binding region of
NusA (riniino acids 1 - 137 } and RNA polymerase is clearly essentiid for function because the
loss of thc rimino-terminal RNA polyrnerase-binding region results in the inability of NusA
to participatc in termination and antiterrnination (Chapter 2 ) . It is Iikely that one or both of
the tuvo large RNA polymerase subunits contact the amino-terminal RNA polymera-se-
binding rcgion of NusA. 1 found that oc alone does not bind the amino-terminal region of
NusA. Also crosslinking data (J. Li and J. Greenblatt. unpublished data) and other evidence
(Liu ct trl. . 1996) suggest that B and P' interact directly with NusA. Neverthclcss. a weak but
imporrrint interriction between a and this region of NusA crinnot be ruled out. hècause the
interaction bçtween a and full length NusA may be strongcr than the interaction between oc
and the carboxy-terminal region of NusA.
1 also showed that N interacts with this same carboxy-terminal autoinhibitory region
of NusA. Thcrefore. I propose that N activates the RNA-binding activity of NusA in a
nirinncr similar to that of a. as modelled in Fig. 76. In this scheme, the close proximity of
NusA to hot11 the r i u t site and N could result in exclusive binding of NusA to the iutr site
RNA. S~ich an interaction might then serve two purposes. First. the interaction of NusA
with m t r site RNA would prevent pause and termination sequences in the nascent RNA from
binding to NusA. This would prevent NusA from enhmcing pausing and termination.
Second. the NusA-n~rr site interaction. together with interactions involving othcr Nus factors.
N. R N A polymerase, and the rzut site RNA wou1d increase the overail stability of the
anti terminrit ion complex containing N.
Friedman and collerigues have shown that a point mutation in the a-CTD or deletion
of the en tire u-CTD enhances antitermination in vivo (Schauer er cd.. 1996). Furthermore.
c\+cn though loss of the a-CTD prevents NusA from stimulating termination irl 17ilt-o. it hrts
no efkct on the abiIity of NusA to stimulate antitermination mediated by the N protein iri
i - i r r o (Li11 CI d.. 1996). This data suggests that there may be a cornpetition between a and N
for binding to the carboxy-terminal region of NusA. Interaction of a with NusA may direct
NusA to thc nascent transcript near its 3' end and facilitate pausing and termination. whereas
interaction of N with NusA woutd block the interaction between NusA and u- direct NusA to
the rurr site RNA. and prevent NusA from stimulating pausing and termination.
Materials and methods
Plasmids, strains and reagents
RXA polymerase. NusA. GST-NusA proteins. GST-N. His,-tagged NusA proteins. a
and a,;, u w e purified as previously described (Chapter 2: Burgess and Jendrisrik. 1975;
Grernblatt and Li. 198 1 b: ~Mogridp et al.. l998a; Mogridge er rd.. 1998b). The plasmids
His a \vt and His a,, for a and %,(a-NTD). respectively. were kindly providrd by K.
Severinot.. Purified k, \vas provided by G. Zhang and S. Darst. NusA R 199.4 was
provided by Ying Zhou and David Friedman. Purification for this protein \vil1 be described
slsewhcrc tT. Mah. Y. Zhou. N. Yu. J. Mogridge, E. Olsen. J. Greenblatt and D. Friedman.
unpublishcd data).
Thc oligonucteotides used for cloning were purchued from ACGT Corp. (Toronto).
RNAguar-d \\.as bought from Pharmacisi Biotech. Restriction enzymes and DNA ligase w r e
purchriscd t'rom New England Biolabs. T7 RNA polymerase was obtained froni Life
Technologies.
Construction of GST-NusA Proteins
PCR primers werc designed to amplify fragments of NrtsA from the pliisrnid J 1 150.
Forum-d and reverse primcrs contained BamHI and EcoRI restriction sites. rcspcctively. !or
wbscqiicnt cloning into the vector pGEX-ZT (Pharmacia).
Purification of GST-NusA Proteins
The Ecoli strain DH5a (Life Technologies) containing the GST-NusA Iiision
plasmids u x grown in 1 i of LB medium to an A, of O S and induced for 3 ho~i r s with 0.5 M
isopropy 1- l -t hio-p-D-galactopyranoside. Ce Ils were hawested by centrifugation.
resuspcndcd. and sonicated in IO ml of 1 M NaCl Buffer A (20 mM Tris-HCI. pH 7-8.0.2
mM EDTA. I mM dithiothreitol. 1 rnM phenylmethylsulfonyl fluoride) and then cenuifuged
for 20 minutes at 12.000 rpin in a Sorval SS34 rotor. Glutathione-sepharose 1B bcads ( 1 ml:
Pliarmaciii) were added to the supernatant and this slurry was rotated for 1 hour üt 4°C. The
brads ivcre ivashed and resuspended in 500 pl thrombin cleavage buffer (50 mM Tris-HCI
pH 7.5. 150 mM NaCl. 2.5 mM CaCL). Thrombin was added to the slurry and incubated at
room tcnipcrature for 1 hour. After centrifugation at 3000 rpm in a tabletop eppendorf
microfugc, the supernatant was collected and didyzed into O. 1 M NaCl ACB ( 10 mM
HEPES pH 7.0. 10% glycerol. 0.1 rnM EDTA, 1 rnM DTT).
Affinity C hromatography
Espcriments with GST. GST-N. and GST-NusA (303-495) were performed as
previotisly described (Chapter 2 of this thesis) except that the GST and GST-N columns were
either 1oadt.d with E-coli extract containing NusA or NusA fragments cleaved from GST kvith
thrombin and the GST and GST-NusA (303-495) columns were loaded with ri mixture of a.
; L I I ~ CC,-,,. buffer and 0.2 m g m l insulin.
Espcriments with the a subunit of RNA polymerase were done two ways. In the first
crisc. u \\.as coupled to aftlgel 10 matrix at three different concentrations. 20 ni1 of beads
nvcrc addcii to 200 ml pipette tips that contained 10 pl of 2 12-300 micron @ass beads
(Sigma). Thc column bed was washed with 10 column volumes of 1 M NaCl ACB (10 rnM
HEPES [pH 7-01. 10% glycerol. O. 1 mM EDTA, 1 m M DTT) and then ~ ~ a s h e d with 10
coliimn i*olumes of 1 0 0 mM NaCl ACB. Columns were loaded with a mixturc of his-tagged
SusA dclciion proteins as ive11 as full length his-tagged NusA. buffer and 0.2 niglrnl insulin.
In the second case. his-tagged NusA ( 1395). GST-NusA ( 1 - 137) and GST-NusA (303-395)
werc couplcd to affigel 10 at equimolar concentrations. 20 ml of beads were added to 200 ml
pipette t ips. The columns were treated as described above except that these columns were
loaded with buffer containing a and 0.2 m g / d insulin.
Gel Mobility Shift Experiments
Gcl rnobility shift assays were performed as previously described (Mogridge et al..
1995) cxccpt that the proteins were added together and incubated on ice for 20 minutes.
Rridio1:ibclIc.d probe was then added and incubation on ice continued for an additional IO
minutes. Concentrations of proteins are indicated in the figure Iegends.
Chanter 4
Intcrriction between the E-coli NusA elon~ation Factor and the r r r r r site RNA
A bstract
Thc mrr site (bo.\il. iirrerbox and bosB) is a cis-acting RNA elemsnt necessary irt i f i w
for antitcrmination by the h N protein. N binds directly to ho-ri? whereas h o d is the
postulated interaction site for the Exoli CO-factors for antiterrnination NusA. NusB and S 10.
By using a series of clustered substitution mutations in bosA and the inter/m.r in gel mobility
shi ft assays. 1 have found that the interho-r and the 3' end of OosA are important for the ability
of NusA 10 associate with an N-nrit site complex. Moreover. the sequence requirements arc
siniilar for binding of wild type NusA to an N-nrrt site cornplex and for the binding to rtur site
RNA of NwA ( 1 - 4 16). ri carboxy-terminally truncated form of NusA that crin bind RNA in
the abscncc of N. As well. the effects of point mutations in /mrB on RNA-hinding by NusA
( 1-4 16) in the absence of N suggested that the formation of a GNRA tetraloop-like structure
in hoxB RNA is important for the binding of NusA to rrrri site RNA. The inribility of NusA
u-i t h ri riiutiition at R 199 in its S 1 homology region to bind to an N-rtrrr site complex indicated
that the S 1 homology region of NusA is important for the association of NusA with RNA. In
contrrist. the 944 mutation in the S 1 homology region aliowed NusA to bind the N-itrti site
complcs. but blocked the sribsequent association of the resulting complex with RNA
polymcriisc. NusB, NusG. and S 10. The ribility of NusA 944 to inhibit the formation of a
complctc complex containing NusA R199A indicated that NusA norrnd1y binds to the N-ilut
site conip1cs prior to the assembly of the other factors.
introduction
Transctiptional antitermination mediated by the N protein of bacteriophage k requires
severrtl Ecoli host factors and a cis-acting element known as the nrir site. The host factors
are callcd Nus. for N-utilization substance. and include NusA. NusB. NusG and S I O
( rcvicwed in Friedman, 1988; Greenblatt et al., 1993). Thesc factors. alon, 0 ~ v i t h the N
protein and the met site RNA. form a stable complex that modifies RNA polymerase into a
termination-resistant form that persists over several kilobases of k sequence and through
niany terminators in a process known as "processive antitermination" (Das and Wolska.
198-1: Mason et al.. 1 W2b: DeVito and Das. 1994; Mogridge rr of.. 1995). Many protein-
protein and protein-RNA interactions have been identified that are likely to stabilize this
mtitci-mination complex.
There is indirect evidence to suggest that NusA interacts with hosil or the inrrrbm
sequence that separates boxA from boxB. Genetic observations that mutations in nucleotides
8 and 9 in hos.4 suppressed the effect of the trirsAl mutation on antitermination hy N
s u g ~ ~ s t c d that hTusA may interact with box4 (Friedman er cil.. 1990). whereas gel mobiliry
shift espcrinients showed that inversion of hosA and the itlrerlmt-. in the contest of a wiId
type hosB. prevents the association of NusA with an N-urrr site complex (Mogridge rr crl..
1995). Furthermore, NusA. as part of the transcription complcx. could be cross-linked to
nasccnt RNA srcater than 10 nucteotides in length (Liu and Hanna. 1995). Consistent with
this c\,idencc suggesting that NusA interacts with RNA. sequence cornparisons have shown
that NUSA contains one S I and two KH homology regions. which are putative RNA-binding
domains (Chapter 3: Gibson et al., 1993; Bycroft et cri.. 1997).
There is no direct evidence to show that either the S 1 or KH hornology regions in
NusA arc involved in RNA-binding. However, mutations in the S 1 homology rqion of
NUSA have been isolated and their effects on E.coli growtli and antitermination by N have
bsen assesscd. The nusAl mutation in this region prevents antitermination by N at 42OC
(Friedman. 197 1 ) and prevents NusA from binding to an N-tz~tr site comples (Mogridge et
LI/.. 1995 ). rz~r.sA RI 99A hris a mutation of a conserved arginine to alanine at position 199 in
the S 1 hon~ology region and tzrrsA R!99K hrts a mutation at the same position. but in this
case. thc arginine was replaced by lysine (Y. Zhou and D. Friedman, unpublished results).
Substitiition of an alanine or lysine for arginine at this position has minimal cffect on the
viabilit), of Ecoli and antitermination by N. Conversely. a strain containing rz~rsA 944. a
Iiybrid belnwn nusr\" and t t r r s ~ ' ~ in which 4 amino acids from E-coli NusA arc replaced by
9 arnino acids from S.rjplzitnrtri~rm NusA in the only major rcgion of heterogeneity between
[he tn.0 proteins. is defecti~~e in supporting antitermination by N. The 944 substitution is also
located in thc S 1 homology region. Interestingly, nrrsA 944 is dominant to rrmA R199A in
the scnsc thrit it prevents antitermination by N in the presence of ntrsA R199A. which would
othcrnVisc allow antitermination (Y. Zhou and D. Friedman. unpublished data).
Gcl mobility shift experiments with a collection of point mutations in cach nucleotide
in rrrt /)O.VA. a sequence that directs antitermination in E-coli 1-171 operons and differs from ic
hmA at positions 1,8 and 9. mapped the binding site for thc NusB/S 10 heterodimer to the
I m . ~ 4 scquencc UGCUCUUUACACA (NodwelI and Greenblatt. 1993). A siniilar mutational
st ticiy sliourcd that the sequence GCUCUU near the 5' end of the boxA scqucnct:
CGCUCVUACACA in the h tziirR site is important for the binding of NusB. S IO. and NusG
to n compicx containing N. NusA, RNA polymerase and the ritrt site RNA (Mogridge et cil..
1998b). Ribosomal protein S 1. a protein consisting of six S 1 domains, ülso binds specifically
to rnr h0-~4 RNA (Mogridge and Greenblatt. 1998). Whereas S 1 crin compete with
NusB/S IO for binding to howî, the nucleotides most important for the binding of S 1 to f>o-\;4
RNA are located mostly toward the 3' end of bm4, at positions 3, 8.9, 10. and 12 (Mogridge
and Grccnblatt, 1998).
G i ~ m the genetic observations that had connected NusA with 60x4. my goal was to
use gcl niobility shift assays to test whether there was likcly to be a direct interaction
between 3usA and b0.a. In previous studies, 1 used a series of three- and fiw-nucleotids
substitution mutations in hosA and the irrrerbox to identify a region of the nrrt site that was
in-iponrint for the association of NusA with an N-rrrrt site complex. These rcsults revealed
that thc 3' end of bo.d and the irirerbox region were most important for the binding of NusA
(T. .Mah. .Masters Thesis. 1995). In the present study. 1 have used these same substitution
mutations to characterize the binding to rrut site RNA of a carboxy-terminülly truncated forrn
of NusA. SiisA ( 1-416). that can bind m i r site RNA on its own (i.e. in the absence of N). 1
have al30 tiscd NusA proteins with mutations at R 199 in the S 1 homology rcgion of NusA to
pro\.ide t.\.idcnce that this predicted RNA-binding region is important for thc interaction
between 'IiisA and RNA. iMoreover, experiments with the 944 mutation in the S 1 homology
region hri\.é énabled me to conclude that NusA nonnally associates with the N-nitr site
çomples prior to the association of this complex with RNA polymerase and the other Nus
frictors.
Results
In\wsion of the l~o.\-A and interbox nucleotides in the r i l i t site prevents the association
of NusA ivith an N-nrtt site complex in a gel mobility shift assay (Mogndge cJr d . . 1995). In
order to tùrther investigate the relationship between this portion of the ruit site and NusA. 1
made ri series of three nucleotide substitution mutations along bosA and a five nucleotide
substitution mutation that altered the entire irlterbox region. I then used these mutated mit
site constructs as RNA probes in gel mobility shift assays. The results from thcse
cxperin-ients were published in my Masters thesis and a sumrnary of them is stiown in Table
1 . .My rcsrilts suggested that the nucleotides important for the association of NtisA with an
N-tilrr site complex are located in the region of bo-vl starting at nucleotide 7 and extending
into the itlrCrIiox region.
Full lcngth NusA does not associate with nltt site RNA on its own ( M o ~ r i d g e et d..
1995). Rather. i t requires interaction with N or the a subunit of RNA polymcrasc in order to
activate its RNA-binding ability (see Chapter 3). Howevcr, a carboxy-terminally truncated
torm of NusA. NusA ( 1 - 4 16). is capable of independcnt association with nrrt site RNA and
this interaction is severely compromised by inversion of the h o ~ 4 and inier1io.v sequences
(sec Cliliptcr 3). To determine whether the nucleotides that are important foi- thc association
of NusA with an N-rirtt site complex are similar to those that are important for NusA ( 1-4 16)
to bind thc r i l r r site on its own, 1 perforrned gel mobility shift experiments with NusA ( 1 - 4 16)
using thc thrce nucleotide substitution mutants of boui and the five nucleotide substitution
mutant of the interbox as RNA probes (Figs. 1 and 2). The binding of NusA ( 14 16) to the
I r i r t site RNA was much weaker than the binding of intact NusA to an N-rzrtr site complex.
NUSA (1-416) shift - Free - RNA
NusA (1 -41 6)
Figure 1. Substitution of nucleotides 1 to 3 in boxA does not affect the ability of NusA (1-416) to interact with nut site RNA, whereas substitution of nucleotides 4 to 6 or nucleotides 7 to 9 substantially reduces the interaction. Reactions containing 0.8, 1.6, or 3.2 pM NusA (1 -41 6) and 32P-labeled wild type or mutant nut site RNAs (as indicated) were electrophoeresed on non-denaturing gels, dried, and exposed to film. Positions of free and complexed RNAs are indicated.
NusA (1-416) shift -
Free - RNA
Figure 2. Substitution of nucleotides 10 to 12 of boxA and 13 to 17 of the interbox both severely affect the ability of NUSA (1 -41 6) to interact with nut site RNA Reactions containing 0.8, 1.6, or 3.2 pM NUSA (1 -416) and 32P-labeled wild type or mutant nut site RNAs (as indicated) were electrophoresed on non-denaturing gels, dried, and exposed to film. Positions of free and cornplexed RNAs are indicated.
This result suggests that the presence of N may stabilize the interaction between NusA and
RNA. Xevertheless, the nucleotides in bu-rA and the iriterhox that were important for the
binding of NusA ( 1-4 16) to r w site RNA were similar to those that were important for the
bindins of NusA to an N-iint site complex. Inversion of ho-ul prevented association (see
Chapter 3 ). mutation of the 5' end of bosA had either no effect o r minimal effect. and
mutations of the 3' end of box4 frorn nucleotides 7 to 12 and the interhm had more profound
effects on thc binding of NusA (1416) to the RNA probe (Table 1). 1 concludc that the
interaction of NusA with an N-nrrt site cornplex is likely to involve a direct interaction of
NusA t\.ith nucleotides in the bo-rA and interbox regions.
Sincc niutations at position 3 (boxB A24C) and 4 (ho.rB A26C) in the loop of boxB
preL7ent the association of NusA with an N-rzrit site complex (Mogridge et cil.. 1995). I
wüntcd to dctermine whether the same nucleotides were important for the binding of NusA
( 1-4 16 to the mir site RNA (Figs. 3 and 4: Table 1). Surprisingly. the binding of NusA ( 1 -
11 6) to hoth the A24C and A26C bosB mutants was only marginally reduced. Nucleotides
(323 and A17 forrn a sheared G-A base pair that is essentiül for forming the GNRA tetraloop-
likc structure in ho-:B (Cai C r cil., 1998: Legault et cil., 1998). Mutations thai af'fccted the
ability o f /mrB to forrn the GNRA tetraloop-like structure. namely buxB G23A and hosB
X 7 C . grcatly inhibited the ability of NusA (1-416) to bind the RNA. Thus. the binding of
NusA ( 1-4 16) to mir site RNA requires specific nucleotides at the 3' end of lzo.\il and the
iiitsfios. and also requires the formation of a GNRA tetraloop-like structure in hasB-
To malyze the involvement of the S 1 homology region in this interaction between
NusA and i l l i t site RNA, 1 made use of three NusA mutants. NusA R199A. NusA R 199K and
NusA 9.44, that were altered in the S 1 domain. These proteins were used in gel mobility shift
NusA (1416) shift
NusA(1-495) - + - - - - + - - . - + - O
NusA (1 -41 6) d d 4
Figure 3. Substitution of nucleotide 23 in the loop of boxB reduces the ability of NusA (1 -41 6) to interact with nut site RNA whereas substitution of nucleotide 24 has little effect. Reactions containing 3.2 pM NusA and 0.8, 1.6, or 3.2 pM NusA (1 -41 6) and 32P-labeled wild type or mutant nut site RNAs (as indicated) were electrophoresed on non-denaturing gels, dried, and exposed to film. Positions of free and complexed RNAs are indicated.
F m - RNA
Figure 4. Substitution of nucleotide 27 in the loop of boxB severely reduces the ability of NusA (1-416) to interact with nut site RNA whereas substitution of nucleotides 25 and 26 has little effect. Reactions containing 3.2 pM NusA or 0.8, 1.6, or 3.2 pM NusA (1 -416) and 32P-labeled wild type or mutant nut site RNAs (as indicated) were electrophoresed on non-denaturing gels, dried, and exposed to film. Positions of free and complexed RNAs are indicated.
nut site variant
wild type
reverse boxA
~ O X A 1-3
~ O X A 4-6
~ O X A 7-9
~ O X A 10-1 2
~ O X A 13-17
boxB G23C
boxB A24C
boxB A2SC
boxB A26C
boxB A27C
N-NusA-nut site NusA (1 -41 6)-nut site
Table 1. Cornparison of the abilities of NusA (1416) , in the absence of N, and wild type NusA, in the presence of N, bind to wild type and mutant nut sites. A plus sign indicates that the nut site mutation has less than a two-fold effect on the formation of either the N-NusA-nut site complex or the NusA (1 -41 6)-nut site complex. A negative sign indicates that there is a three- to five-fold reduction in the formation of the complex. ND indicates that an experiment with that particular mutant nut site was not done. a Mogridge et al. 1995 b T. Mah, Masters Thesis. 1995
assays with N. RNA polymerase, and the rest of the Nus factors to assess the impact of these
mutations on the formation of various complexes containing these factors (Figs. 5-8). The
addition of 3i protein to wild type nrrt site RNA resulted in the formation of an N-rzrrr site
comples (Fig. 5a. Iane 2: Fig. 5b, lane 2; T. Mah, Masters Thesis. 1995; Mogridge er al.,
1995). while the addition of increasing amounts of NusA to this cornplex furthcr retarded the
mobility of the RNA, resulting in the formation of an N-NusA-nrrt site complex (Fig. 5a,
lancs 3-5: Fig. 5b. lane 3: T. Mah. Masters Thesis. 1995: -Mogridge et ni.. 1995 1. Howi-ver.
NusA R 199A and NusA R 199K were unable to cause this supershift of the N-rrlrr site
complêx (Fig. 5a. Ianes 6- 1 1 ).
A consensus rrrrt site which has alterations in nucleotides 8 and 9 that makes h l~osA
rcsemblc r r r i h o d was able to suppress the effect of the rrlrsAl mutation on üntitermination
by N (Friedman et cri.. 1990). It was suggested that the alteration of nucleotides 8 and 9 in
ho.d tvor~ld i mprove the interaction between NusA and h o - ~ 4 (Friedman et trl.. 1990).
Ho\vc\.cr. LISC of the consensus mir site RNA as a probe with the RI99 mutant NusA proteins
did not improve the abiliiy of these proteins to associate with an N-nlrr site complex (Fis. 5a.
Ianes 16- 19). Although thé NusA mutants altered at position 199 are unable to bind an N-/ tu t
site coniplcs. NusA 944 was able to supershift an N-nlct site complex (Fis. 5b. Iane 4).
These sxpcriments suggest that the conserved arginine at position 199 is important for the
~ibility 01' KiisA to interact with an N-nrrt site complex but that not al1 mutations in the S 1
homology rcgion result in this type of defect.
.Mutation of the arginine at position 199 of NusA did not alter E-coli i-iability nor did
i t riffcct mtitcrmination by N. I-iowever. substitution of nine S.r~phit?r~rritrnr amino acids for
four E c - o f i amino acids in the 4-for-9 region resulted in a NusA 944 protein defcctive in
NONUSA shift - N
shift --)
tree ANA -
LL wild type nutsite consensus nut site wild type nut site
N + + + + + + + + + - + + + + + + + - + + + NUSA - - + ++ +++ - - - - - - - - ++ +++ - - - 1 - + -
NUSA R199A - - D o - + ++ +++ - - - +++++, O 9 0 O I
NUSA R199K - - - - - I o - + + + + + + - - - - - +t +t+ 1 - 1
Figure 5. Mutations in the SI homology region of NusA have differential effects on the formation of N-NusA-nui site complexes. (a) Mutation of position 199 in NusA prevents the formation of an N-NusA-nui site complex. (b) Alteration of the 944 region does not affect the formation of an N-NusA-nut site cornplex. Reactions containing 32P-labelled wild type or consensus nut site RNAs and various combinations of 1500 nM N, and either 400 nM (+), 600 nM (++), or 800 nM (+++) NusA or NusA mutants (as indicated) were electrophoresed, dried and exposed to film.
omplete complex -nut site complex mut site cornplex
RNA polymerase - + + + + + + + + + + NUSA wt - - + + - - O - + + -
NusA R199A - - + + - - + - + NUSA 944 - + + + +
N - - + + + + + + + + + BIG 1s - O - + - + - + + + +
Figure 6. Mutations in the S 1 homology region o f NusA have differential effects on the formation o f a complete complex containing RNA polymerase. N. NusA. NusB. NusG and S 10. Reactions containing 32~-labelled nicf site RNA and various combinations of 500 nM N, 5 0 nM NusB, 50 nM NusG. 50 nM S 10, 25 nM RNA polymerase and 100 nM NusA or NusA mutants (as indicated) were elcctrophoresed on 5% non-denaturing gels, dried and exposed to film.
supporting antitermination by N (Y. Zhou and D. Friedman. unpublished results). This is
precisely the opposite of what might have been predicted on the basis of the effects of these
mutations on the binding of NusA to N-nut site complexes (F ig 5 ) . To test whether these
mutant proteins were able to assemble into complete complexes containing RNA polymerase.
1 perforrned sel rnobility shift experiments using RNA polymerase. N, NusA. NusB. NusG
and S 1 O (Fig. 6: Chapter 2: Mogridge et al.. 1995). RNA polymerase bound to RNA on its
okvn and thc addition of N and NusA resulted in the formation of an N-NUSA-RNA
polymcrrisc-nlrt site complex (Fit. 6. lanes 2 and 3). Addition of NusB, NusG and S 10. as
well as N. XusA, and Rhr.4 polymerase. resulted in the formation of a completc complex of
Ion- rnobility (Fig. 6. lane 4). Despite the tact that NusA R199A could not interact with an
N-rlllr site cornplex (Fig. 5a) and only allowed formation of it srna11 ;unoi.int of an N-NusA-
R N A polymerase-nur site complex (Fig. 6. lane 5). it could efficiently support the formation
of a coniplett' complex containing al1 of the added factors (Fig. 6. lane 6). This result was
consistent ni th the observation that NusA R199A supports antitermination by N. ffowever.
NusA 944 u.as unable to support formation of the completc complex (Fig. 6. Ime 8). evcn
though i t cnuld associate \vith an N-nut site compIex (Fig. 5b. lane 4). thus esplaining why
rrusA 944 does not support antitermination by N. Funherrnore. in a reaction in which an
cqual amount of NusA R199A was mixed with NusA 944. NusA 944 was able to prevent
SusA R i 99A from assembling into a complete complex ( Fig. 6. lane I 1 ). In contrast. NusA
914 allo\vcd formation of a complete complex when wild type NusA was present (Fig. 6.
lane 10 1. Thcse results probably explain why the nitsA 944 mutation is dominant to nrrsA
R199A. but recessive to wild type nusA.
Discussion
1 have shown that mutation of nucleotides in the inrerbox o r the 3' end ofho-vî reduce
the association o f NusA ( 1 - 4 16) with mit site RNA in the absence o f N or thc association of
wild type NcisA with an hi-rtnr site cornplex. These results strongly sus tes t that NusA
interricts directly with this region of the nrtr site RNA. A similar conclusion wu reüched on
the b a i s of the observations that mutation in nucleotide 8 of hox4 allows Tor the functioning
of the S.rydtiurrtrirtnz NusA with h (Friedman and Olson. 1983) and that mutations in
nucleotidcs 8 and 9 suppress the effect o f the nusAl mutation on antiterrnination by N
(Fricciman cr (ri.. 1990). However, 1 found that changes in hoxA nucleotides 8 and 9 d o not
affect thc binding o f NusA (Fig. 5a) and Nodwell (Nodwell and Greenblatt. 1993) found that
thcsc niutations stabilize the binding to rrn bo-vl RNA of NusB and S 10. Thus. NusA does
interact nri th /~oOuA and irrtc.rhox RNA, but the interaction does not involve hosA nucleotides 8
and 9. Changes here most likely indirectly suppress the nir.sA/ mutation by snhancing the
bindin: 01' XLISB and S 10. Other studies have suggested that hox4 is not rcqiiircd for
rintitermination over a short distance, but rather that it is pan of a regulatory systcrn that
binds an inhibitor o f antitermination (Zuber et al., 1987; Patterson er crl.. 1994 1.
Nevcrthelcss. it is likely that this region in hoxA and the irirerhox defines a low affinity
binding site 1Or NusA. and that there is a second low affinity site in hosB.
Thc nucleotides in h hosA and the ir~terbo-r that arc important for NusA interaction
are sirnilar but not identical to those that are important for S 1 interaction with m i h0.d
(Mogridgc and Greenblatt. 1998). Funhermore. S 1 binds to h hmA (Mogridgc and
Greenblatt. 1998). Since S 1 consists of six S 1 domains. it is possible that the S 1 domains in
both S 1 and NusA recognize sirnilar RNA sequences. Based on these observations, it would
bc rcrisonriblc to suggest that the S 1 hornology region in NusA binds ho-d . whcreris the KH
homology regions in NusA recognize nucleotides in bo-rB.
Full length NusA does not bind mtt site RNA o n its own in gel mobility shift
txperimencs (Mogridge err (11.. 1995). The binding of NusA ( 1-116) to the wild type nrtr site
RNA indicrites that NusA is capable of interaction with RNA and suggests that rernoval of
the carbosy-terminal region of NusA relieves inhibition of RNA-binding. Esperiments in
Chapter -3 ot' this thesis show that the binding of N o r a to the carboxy-terminal region of
XusA allows interaction of NusA with RNA. Together. my results strongly support the
suggestion that the carboxy-terminal region of NusA inhibits RNA-binding.
Thc hosA and inter/ms- nucleotides that are important for the association of NusA
with an S-firrr site comples are also important for the association of NusA ( 1 -4 16) with rilrt
sitc RNA in the absence of N. However. nucleotides in the loop of bosB differ in their
imponancc for the formation of these complexes. These results suggest that these two
d i k r c n t fornis of NusA bind boxf3 RNA differently. It is possible that the RNA-binding
r q i o n s of' NusA that are cxposed in NusA ( 1-416) are not in the sarnc configuration as thcy
are when NiisA binds the trur site in an N-nltf site cornplex. The binding of NusA ( 1-416) to
thé r r r r r hite R N A required the formation of the GNRA tetriloop-Iike structure. but the
identitich O!' the three interna1 loop nucleotides were not important. In contrist. wild type
SusA cocild not associate with an N-ncct site cornplex if nucleotide 2 or 4 of thc loop was
mutritcd (.Mogridge et al . . 1995). It is likely that bo.rB RNA crin exist in altcrnative
conformations since it forms difîèrent GNRA tetraloop-Iike structures depending on whether
ihc N protcin from h o r Pz2 is bound (Cai et al.. 1998; Greenblatt et al.. 1998: Legault er cd . .
1998). Thus. it is possible that N traps boxt? in a conformation recognizable by NusA that
c m only be formed when the N protein is present. If this is correct. full length NusA
wouid intcract with the exiruded loop nucleotide 4 in the N-hoxi3 complex. whereas this
nuclcotide urouid have a different conformation in the absence of N.
My expeliments with NusA variants in the S 1 homoiogy region suggested that at least
the conscn.ed ürginine at position 199 of the protein is important for RNA-binding. This
mutation hm oniy a mild effect on antitermination by N hl i-i iw. yet prcvents NusA from
interacting \vith an N-mrr site complex. This apparent contradiction was resolved by an
csperirnent in which RNA polymerase and the other Nus factors were added to the reaction.
NusA R I99A did allow the formation of a complete complex containing N. RNA
polyn-icrase. and the other Nus factors. Thus. other protein-protein and protein-RNA
i ntcractions within the N-modified antitermination complex can compensats for a weakened
interaction of NusA with the rrirt site RNA.
These studies also provided an indication of the normal order of assembly o f the
antitermin;ition complex. The NusA 944 mutant protein was able to associate with the N-rzur
sitc complcs. but was unable subsequently to assemble into a complex containing RNA
polyrncrrisc and the remaining Nus factors. NusB, NusG. and S IO. Moreover. the NusA 944
protcin prcvented the formation of a complete complex containing NusA R 199A. indicating
that ihc X-NusA-rurr site complex must normdly assemble first. with the other hc to r s joinins
thc coniples later. Sincc the rirtsA 944 mutation is also dominant to rzrrsA R199A in vivo (Y.
Zhou and D. Friedman. unpublished data). it is likely that NUSA also associates first with the
N-~llrr sitc complex itl i?i\.o prior to the assocation with the other Nus factors and RNA
polymcrrise.
Materials and Methods
Plasmids, strains and reagents
R N A polymerase. N. NusA. NusA ( 1-493, NusA ( 1-4 16). NusB, and NusG were
piiri!?cd as descnbed previously (Chapter 3; Burgess and Jendrisak. 1975: Grcenblatt et tri..
1980: Grccnblatt and Li, 198 1 b; Swindle et al-, 1988: Li et c d - , 1992). Nusii R I W A and
NusA 911 were provided by Y. Zhou and D. Friedman. S IO was provided by V. Nowotny.
Gel mobility shift experiments
Gcl mobility shift assays were pcrformed as previousty described (T. .Mith. Masters
Thcsis. 1995: ~Mogridge C r cd.. 1995). Concentrations of proteins are indicated in the figure
Icgcnds. TIK sequences of the wild type r z r r t site and the mutant rrirt sites arc as tollows:
pKUT n.ild type: 5' CGCUCUUACACALXJCCAGCCCUGAAAAAAAGGGC -3'
pXUT 1 -i: 5' GCGUCULTACACAUUCCAGCCCUGAAAAAGGGC 3'
pXLT 1-0: 5' CGCGGGUACACAUUCCAGCCCUGAAAAAGGGC 3'
pNUT 7-9: 5' CGCLrCUGCGACAUUCCAGCCCUGAAAAAGGGC 3'
pNUTrc\.: 5' ACCUUACACALTUCUCGCGCCCL'GAAAAAGGGC 3'
phiUTcon: 5' CGCUCUULTAACAUUCCAGCCCUGAAAAAGGGC 3'
The RNA probes contain additional vector sequences and are 5.1 nucleotides long.
Th i sis irectio s
In order to understand the ways in which the NusA elongation factor crimies out its
functions in elongation. termination and antitermination. 1 have malyzed protein-protein and
protein-RN A interactions involving NusA. A mutational study of b& and the interbox
cstablished that nucleotides at the 3' end of 60-~4 and the Nzrrrbox are most important for the
interaction of the h nrtr site R N A with NusA. A systematic deletion study on NusA ailowed
rh r idcntilicntion of regions of interaction with RNA polymerüse and the À N protein. and
also pro\.ided evidence that the S 1 homology region in NusA is important for its interaction
with rrl t t site RNA. The t ~ v o RNA polyrnerase-binding rezions of NusA were localized to
iimino acids I - 137 and 232-495. A carboxy-terminal region of NusA. from anlino acids 303-
495. bound the a subunit o f RNA polymerrise, and this region of NUSA also interricted with
the À .I' protein. The identification of a carboxy-terminally truncated forrn of NusA that
could bind RNA on its ourn further supported the proposai that NusA interacts tiirectly with
RNA. FinalIy. 1 developed a model to expiain certain aspects of NusA function (Chaptcr 3.
Fig, 7 ) . The tollowing is a discussion of what additional experiments couid bc done to
confirrn and add to the model.
In Chapter 4. 1 showed that the 3' portion of bosA and the interimx arc important t'or
the association of NusA with rtiit site RNA. Ribosomal protein S 1 binds to a sirnilu region
in rriz ho.vi\ and also interricts with h ho-UA. although the exact region was not determined
(.Mogridgc and Greenblatt. 19%). These observations suggest that the S 1 domains in the S 1
protein and XusA may interact selectively with a similar binding site in bosA RNA.
Howcver. S 1 cornpetes with the NusB/S IO heterodimer for binding to rrrl i7o.t-A RNA. and
has no ctlcct on non-processive antiterrnination by N, which depends only on N and NusA.
or ml-mediated antiterrnination (Mogridge and Greenblatt. 1998). Since NusA is required
for both of these processes. S 1 probably does not compete wiih NusA for binding to
nrhcn other Fxtors are present to aid the association of NusA. However. it would be of
interest to determine if NusA and S 1 can bind to box4 at the same time by performing gel
rnobiIity shifi experiments as well as RNA pull-down expcriments. Since both proteins bind
to /MISA. i t is possible that the S 1 domains within the proteins are interchangeable. To
determine u.hether this is true. the S 1 domain of NusA could be replaced with one from the
S 1 protein and the resulting NusA mutant could be tested for function in termination and
rintitcrn~ination. However. only a positive result would be meaningful in this type of
cxperin~ent.
Aniple evidence to support the theory that NusA does interact directly with RNA and
that /md and the i n r e r h ï are important for the interaction wüs provided by my observations
that the iibility of NusA to associate with an N-ririr site complex is affected by mutations in
hoxA and thc i~lrerbos and that NusA ( 1-4 16) can bind RNA on its own (Le. in the absence of
Ni . Whcrcas genetic experiments had suggested that box4 could be the binding site for
SUSA. otht's cxperirnents had suggested thrit box4 is unimportant for NusA function (Olson
cf trl.. 1982: Friedman and Olson, I983; Olson et cd . , 1984: Zuber et cil.. 1987: Patterson cJr
(11.. i 994). Aithough I havc shown that /~osA and the irzrerhox represent an important binding
site for NusA. there are probably other important binding sites that promote NusA function
when thc nrrt site is not present. T o identify other important sequences for NusA binding.
one could do an experiment where the mir site RNA is randomized and the rcsulting pool of'
RNAs is wbjected to selection for binding to NusA (1-4 16) (ie ri SELEX-type experiment).
In addition to identifying ncw binding sites. this type of experiment should contirm the
importance of the wild type rrut site since the nrrt site should also be identified in the
sclcction. A more direct approach could also be used in which RYA sequenccs from pause
and termination sites that are influenced by NusA are used ris RNA probes in gel mobility
shift esperiments. Once new sequences that interact with NusA are identiticd. interactions
u.ith intact NusA in the presence of the a subunit of RNA polymerase would provide
svidcnce for the relevance of the sequences in transcription. As well. the isolated S 1 and KH
homolog rcgions from NusA could be used in binding experiments to deterniine whether
different binding sites couid be identified for each type of RNA-binding domriin within
NusA.
The interaction of NusA ( 1 -4 16) with bu-vt RNA was similar to that of full length
NusA in thc presence of N. Sirniiarly. the non-Watson-Crick G-A base pair in the boxB Ioop
is necessri1-y for the interaction in both cases. but the extruded fourth nucleotide of the bnsB
loop \va'; not necessary for the binding of NusA (1-416) in the absence of N. Therefore. the
fourth nuclcotide of the hnxB loop may hcilitate the interaction of a portion of N with NusA.
Alternriti\.cly. the RNA-bindins modules in NusA may be aligned differently in the two
difièrent SusA constructs. To detemine if this were true. one couid perform limited
proteolysis on both of the proteins and compare the respective proteolysis patterns.
Funhcrrnorc. addition of r i l i r site RNA to the proteins followed by proteolysis would provide
ciritri on \vhc.ther similar fragments are protected from cleavage by the presencc of RNA. The
protcolysis products could be rtnalyzed by rnass spectrometry to further document any
difkrcnccs bctween the two proteins.
S incc the interaction between NusA ( 1-4 16) and the ririt site RNA is wcak. it is not
clear if d l of the RNA-binding regions within the truncated NusA protein are amilable for
RNA-binding. It is possible that the conformation of the protein is not optimal for full RNA-
binding activity. By introducing mutations that prevent RNA-binding into onc or other o f the
RNA-hinding regions of NusA ( 1-4 16). it may be possible to determine which RNA-binding
domriin(s) is active. T w o such mutations in the S 1 domain of NusA have been characterized.
The ~i~r.sA I mutation has a substitution of a leucine for an arginine at position 153 and NusA
R 199A has a mutation of a conserved arginine to an alanine that prevents the association o f
SusA npith iin N-urrr site complex (Mogridge er al., 1995: Y. Zhou. T. .Mah. N. Yu. J.
.Mogridgc. E. Olsen. J. Grcenblatt and D. Friedman, unpublished data). Thus. if this mutation
were introduced into NusA ( 1-416) and it prevented the ability of the protein to bind the t z u f
site RNA. it would sugzest that the S 1 homology region in NusA ( 1-4 16) is nccessary for
binding. Conversely. if NusA R199A ( 1-416) still bound tzrrr site RNA. this would imply that
the KH cloiriains may be the only ones involved in RNA-binding. This could be confirmed
by the L I X of a similar type of mutation in either o f the KH domains of NusA.
In Chapter 2. I present results of a systematic deletion study on NusA. Rcgions that
interact nith RNA polymerase. N and RNA were identified. T o delinerite domain boundaries
niorc prccisciy. limited proteolysis should be done. The resulting fragments of NusA should
rcprcsent intact domains that are not accessible to limiting arnounts of protcasc. This mixture
COLI^^ bc p a s c d over affinity columns containing immobilized RNA polymcrrise or N to
niore prcciscly map the protein-interacting domains. The RNA-binding domains could be
studicd cising the binding assays outlined above. The exact domain boundarics could be
determincd by subjecting the interacting fragments to m a s spectrometry. Thc information
ocneratcd ti-om this type of study could then be used to furthcr study the interactions between t
the protcins éither by NMR. o r by mutational anaiysis (see below).
The interaction between NusA and the )i N protein was studied in Chapters 2 and 3 .
Rcsuits froni these studies suggested that one of the purposes of the interaction between the
carbosy-terminal region of NusA and N is to relieve the inhibition of RNA-binding by NusA
mediritcd by the carboxy-terminal region of NusA. This concept would explain why NusA
i 1-3-18) ufas completely fiinctional in antitennination even though it did not bind N (Chapter
2). The mode1 presented in Chapter 3 proposes that. once inhibition of RNA-binding Ly
NusA is rclieved. either the f i l t t site RNA or the nstscent RNA will be bound hy NusA. Thus.
the other piirpose of the N-NusA interaction would be to bring the i l w site into close
proximity with NusA so that the nirt site. as opposed to nascent RNA. is bound to NusA. Ta
test this idea. one could seek mutations in NusA that prevent the N-NusA interriction. but not
the NusA-u. or NUSA-RNA interaction. Using information generated from the domain
rnapping csperiments proposed above. it would be possible to mutate speciliç amino acids in
thc defincd Y-binding domain of NusA to create a mutant protein unabte to intcrrict with N.
Once puriticd. this mutant protein couId be used in gel mobility shift experiments to
detcrn~inc n.hcthcr the loss of N-binding ability by NUSA would prevent thc association of
SUSA n-i lh an N-itrtr site complex. In terms of functional assays. the prediction is that a
mutant SLKA protein that does not bind N would be unable to enhance antitcrmination by N.
but n-ould still be able to cnhance termination as a resuIt of its interaction with a.
Thus far. the data has been generated with in virro binding and transcription assays.
However. structura1 data could add greatly to our understanding of the functions of NusA.
Thcrcforc. an NMR or X-ray diffraction study of a complex containing NusA ( 132416)
uvhich binds N and RNA. N ( 1-17) which binds bo.rB and NusA. and t m r site RNA would be
of grcat value. Although the entire comptex would be ver), large by NMR standards,
structure dctermination would be simplifled by the possibility of introducing hesivy isotopes
individually into NusA ( 1323 16). N ( 137). and the nrrt site RNA.
Finaliy. a key question for understanding NusA function in antiterminütion is whether
X and a compete for binding to the carboxy-terminal region of NusA. First. the minimal
binding sites for both proteins should be mapprd by the mcthod of lirnited proteolysis as
described ahove. To determine if N and a can bind NusA at the same time. onc could
pcrform ri gradient sedimentation experirnent with mixtures containing various combinations
of NusA. N and a.
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