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Introduction to

Genetic AnalysisTENTH EDITION

Griffiths • Wessler • Carroll • Doebley

© 2!2 W" H" #ree$an and Co$%any

CHA&TE' !!Regulation of Gene Expression in

Bacteria and Their Viruses

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CHA&TE' O(T)INE

11.1 Gene regulation11.2 Discovery of the lac system negative control

11.! "ata#olite repression of the lac operon positive control

11.$ Dual positive and negative control the ara#inose operon

11.% &eta#olic path'ays and additional levels of regulationattenuation

11.( Bacteriophage life cycles more regulators) complex

operons

11.* +lternative sigma factors regulate large sets of genes

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Gene Regulation

, Gene Regulation

, Differential gene expression is important-

, ot every gene is expressed all the time

, "ells have mechanisms that

, Recogni/e environmental conditions0'hen they

should active or repress transcription of genes

,Turn on or off lie a toggle s'itch3 transcription ofeach gene or groups of genes

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Types of Gene Expression

, Genes can #e characteri/ed #y their expression

, 4ome genes are 5on6 all the time 7 these areconstituti*e genes

, 8nclude 5houseeeping6 genes

, Encode proteins that need to #e transcri#ed continually

, Ex. +ctin or u#i9uitin, 4ome genes are transcri#ed only 'hen needed 7

these are facultati*e genes

, Turned on:off in response to change in the cell;s

environment e.g. availa#le nutrients3, <r dependent on the cell cycle

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, Regulation of facultative genes 7 thin of services

, Constituti*e genes

, 5On6 all the time

, Ex =tilities lie B" >ydro

, T'o main inds of facultati*e genes

, Inducible genes

, Gene is normally 5off 6 7 s'itched on 'hen needed

, Ex R"&? or @ire Department, 'e%ressible genes

, Gene is normally 5on6 7 s'itched off 'hen necessary

, Ex "anada ?ost

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, "ontrol of transcription, T'o main 'ays to control transcription of specific

genes

, Depends on re+ulatory %roteins

, &ositi*e control 7 acti*ator %rotein increasestranscription of gene activator #inding site3

, Ne+ati*e control 7 re%ressor %rotein decreases

transcription of gene #ind to operator3

, Regulatory proteins bind DNA near gene, Re9uire specific D+ nucleotide se9uences

, Regulatory proteins are very specific

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Regulatory proteins control transcription

?ositive presence of activator

allo's transcription of gene

egative absence of repressor

allo's transcription of gene

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Regulatory proteins control transcription

, Regulatory proteins are also regulated-

, Effector molecules can regulate activators

and repressors

, <ften small molecules, &ay #e meta#olites

, +ctivate or deactivate regulatory proteins

, Allosteric regulation is common

, Effector #inds to regulatory protein

, "hanges conformation of D+ #inding site of regulatory

protein

, "hanges its a#ility to #ind D+

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+llosteric effectors #ind to regulatory proteins

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Repressor protein controls the lac operon

, The Lac <peron a set of genes3 in E. coli to meta#oli/e

lactose 7 a disaccharide

, &eta#olism of lactose re9uires t'o proteins

, Aac permease lacY 3 7 transports lactose into cell, CGalactosidase lacZ 3 7 cleaves lactose into

glucose and galactose monosaccharides3

, Genes are transcri#ed onto a single mR+

, ?roaryotes can mae mR+ 'ith multiple genes, Aac of introns:exons and mR+ processing maes this

possi#le

, "alled 5polycistronic6 mR+

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, The Lac <peron a set of genes3 in E. coli to meta#oli/e


, Genes are transcri#ed onto a single mR+

, aco# &onod discovered three genes #yclassical genetic approach

, =sed mutant strains of E. coli

, Fe 'ill revisit their experiments later in the

semester

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, The Lac O%eron a set of genes3 in E. coli to meta#oli/e


, The Lac operon is inducible

, Turned on 'hen needed, The Lac operon in under ne+ati*e control

, "ontrolled #y a repressor lacI 3

, Repressor is deactivated #y an effector molecule

inducer 3

, )actose acts as the inducer

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Aactose is #roen do'n into t'o sugars

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The lac operon is transcri#ed only in the presence of lactose

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, Lac operon regulation 7 lactose induces expression

, The lac repressor is encoded #y lacI

, lacI is located some distance from the lac operon

, The repressor #inds the lac operator O3

#et'een the promoter and the lacZ gene, Aactose is an allosteric effector of the lac

repressor

, 8f lactose is present it #inds the repressor

, The repressor changes shape and falls off the D+, R+ polymerase can #ind the lac promoter

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The lac operon

, Experiments using the lac operon

, Aactose is an inducer of the lac operon

, >o'ever) it is #roen do'n #y CGalactosidase

, Difficult for experimentation-

, <ther lactoseCshaped3 molecules may induce

the lac operon

, 8sopropylCCDCthiogalactoside 8?TG3 can #ind to

the lac repressor molecule) inactivating it

, CGalactosidase does not act on 8?TG

, 4'itches on the lac operon) #ut is not used up

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4tructure of 8?TG

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Regulation of the lac operon

, Fhat if lactose and glucose are presentH

, E. coli prefers glucose over lactose

, 8f both are present lac operon is ept

5off6

, + %ositi*e control mechanism is at 'or

, "ata#olite repression 7 a #reado'n

product of meta#olism controls

transcription

, 8f glucose is %resent) AT& is plentiful

, 8f glucose is absent) A,& levels

increase, <ne version of +&? is cyclic A,&

c+&?3

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Glucose levels control the lac operon

R l i f h l

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Regulation of the lac operon

, The cata#olite c+&? controls transcription

, c+&? #inds catabolite acti*ator %rotein

CA&3 c+&?C"+? complex

, c+&?C"+? complex #inds lac promoter , +cts as acti*ator and increases transcription

, +dded level of control ensures proper

transcription < high lactose and lo' glucose

<@@ the rest of the time

Gl l l t l th l

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Glucose levels control the lac operon

& D+ #i di it t i l

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&any D+ #inding sites are symmetrical

4ymmetrical #inding sites mae control easier-

Bi di f "+? # d D+

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Binding of "+? #ends D+

ti d iti t l f th l

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egative and positive control of the lac operon


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egative and ?ositive "ontrol of Transcription

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egative and ?ositive "ontrol of Transcription

, "ells have different 'ays to regulate transcription

Repression and activation compared

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-Generic.


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-Generic.

Dual ?ositive and egative "ontrol

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Dual ?ositive and egative "ontrol

, <ther control systems use co$binations of repression

and activation

, Arabinose O%eron in E. coli

, +ra#inose is a sugar , araB) araA) araD genes polycistronic mR+3

, Encode proteins to #rea do'n sugar

, araI is the initiator re+ion

&ap of the ara operon

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&ap of the ara operon

The ara operon contains several genes

4tructural genes encode proteins

Regulation of the ara <peron

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Regulation of the ara <peron

, +ra" protein controls the ara operon

, The gene araC encodes a regulatory protein

AraC

, AraC is #oth an activator and a repressor , araI is the initiator region of the operon

, 8t affects #inding of R+ pol to the promoter

, Arabinose is an effector molecule

, +ra#inose I +ra" protein #ind araI

, CA& / cA,& also #ind araI

, "+? I c+&?3 I +ra#inose I +ra"3 J lots of R+

+ra" serves as an activator and as a repressor

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+ra#inose operon is 5s'itched on6 #y

+ra" I +ra#inose J lots of ara#inose3

"+? I c+&? J lo' levels of glucose3

Fhat if there is no ara#inose in the cellH


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Gene order in the trp operon corresponds to reaction order in the #iosynthetic path'ay

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Gene order in the trp operon corresponds to reaction order in the #iosynthetic path'ay

The trp operon encodes genes to synthesi/e tryptophan

Regulation of the trp operon

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, trp operon 7 controls synthesis of tryptophan

, trp gene expression is dependent on the

levels of tryptophan trp3 in the cell

, trp operon is under control of a re%ressor, Encoded #y the gene trpR

, The trp repressor is activated #y tryptophan

, 4tops expression of the trp operon #y #inding to

the operator trpO


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g p p

, trp operon 7 controls synthesis of tryptophan

, The trp operon is also regulated #y

attenuation

, Attenuation means 5turning do'n60not all the'ay off

, This mechanism is #uilt into the trp mR+

The trp mR+ leader se9uence contains an attenuator region and t'o tryptophan codons

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p 9 g yp p

T'o trp codons (GG3 in aro' upstream from trp operon

+#undant tryptophan attenuates transcription of the trp operon

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yp p p p p


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g p p

, +ttenuation fineCtunes gene expression

, +ttenuation affects translation not transcription

, The trp operon is an example of ho' regulatory

mechanisms allo' fine regulation of geneexpression

, "ritical for maintaining homeostasis

, +llo's 9uic adKustments to respond to changes in

the environment

, Tryptophan is not the only system 'ith

attenuation of gene expression

Aeader peptides of amino acid #iosynthesis operons

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p p y p

Regulation of gene expression

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, ?hage gene regulation

, Bacteriophage life cycles more regulators)

complex operons

8mage '''./yvexla#s.com

>elixCturnChelix is a common D+C#inding motif

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D+ #inding depends on physical structures

+mino acid side chains determine the specificity of D+ #inding

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