operons chapter 18 campbell and reece. e. coli synthesizes tryptophan 5 genes clustered together...
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OPERONSChapter 18Campbell and Reece
E. Coli synthesizes tryptophan
5 genes clustered together
each reaction in pathway catalyzed by specific enzyme
one promoter (site where RNA polymerase can attach) serves all 5 genes
E. Coli synthesizes tryptophan
transcription of these 5 genes 1 very long mRNA that codes for 5 polypeptides that make up the 5 enzymes in tryptophan pathway◦mRNA is punctuated with stops &
starts
E. Coli synthesizes tryptophan
advantage of grouping genes: can use 1 “switch” controls cluster of functionally related genes: they are “coordinately controlled”
“switch” is a segment of DNA called an operator
Operator positioned w/in or between
promoter & genescontrols access of RNA
polymerase to genes
Operon includes: promoter, operator, &
genes
How does the “switch” work?just by itself: trp operon is in “on”
positionto switch it off: trp repressor
must bind to operator which blocks attachment of RNA polymerase◦repressor proteins are specific for
each operator◦repressors are protein product of a regulatory gene called trpR located some distance from trp operon has own promoter
Regulatory Genescontinuously expressed @ low ratetrp repressor is allosteric (2
alternate shapes: active & inactive)◦trp repressor made in its inactive form
with low affinity for trp operatorrepressor binding is reversible
◦operator vacillates between 2 states 1 w/out repressor bound 1 with repressor bound duration of each state depends on # of
active repressor molecules around
Regulatory Genestrp repressor is allosteric (2
alternate shapes: active & inactive)◦trp repressor made in its inactive
form with low affinity for trp operator◦when tryptophan binds to trp
repressor @ allosteric site protein changes to active form shape can attach to operator turning operon off
Tryptophan functions as corepressor:
◦small molecule that cooperates with repressor protein to switch operon off
◦if cell’s tryptophan levels high more molecules will associate with a repressor protein to switch it off
◦if cell’s tryptophan levels low transcription of operon’s genes resume
trp operon animationhttp://
highered.mcgraw-hill.com/sites/dl/free/0072835125/126997/animation28.html
2 Types of (-) Gene Regulation
1. Repressible Operon◦ transcription usually in “on” position
but it can be repressed when specific molecule binds to regulatory protein
◦ ex: trp operon
2. Inducible Operon◦ usually in “off”position but can be
induced when small molecule interacts with regulatory protein
◦ ex: lac operon
Lac operonlactose: disaccharide in milk
available to E. coli in human intestine when person drinks milk
lactose glucose + galactose◦hydrolysis reaction by enzyme β-
galactosidase◦in absence of lactose E. coli has only
a few molecules of β-galactosidase◦+ milk w/in 15 min bacteria has
1,000s x more
Lac Operon3 genes:
◦1: β-galactosidase◦2: enzymes that function in lactose
utilization
Lac Operonentire transcription unit is under
control of 1 main operator & promoter
lacl: regulatory gene outside of operon codes for allosteric repressor protein that can switch off lac operon by binding to operator
by itself lac repressor active binds to operator switching lac operon off
Repressorachieved by specific small
molecule called an inducer inactivates repressor◦lac operon: its inducer is allolactose
(isomer of lactose: sm amt made when lactose enters cell)
◦no lactose no allolactose lac repressor in active shape genes of lac operon not being transcribed
Inactivation of Repressor+ lactose allolactose binds to
lac repressor it changes shape nullifying repressor’s ability to attach to operator transcription of lactose-utilizing enzymes
Lac Repressor
Inducible Enzymesex: lac operontheir synthesis is induced by a
chemical signal (allolactose)generally, function in catabolic
pathways
Repressible Enzymesex: tryptophan synthesisgenerally function in anabolic
pathways
both repressible & inducible enzymes involves negative control of genes (operons switched off by active form of repressor protein)
http://highered.mcgraw-hill.com/sites/007337797x/student_view0/chapter13/animation_quiz_-_combination_of_switches__the_lac_operon.html
http://www.wwnorton.com/college/biology/mbio/animations/main.asp?chno=ch10a01