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OPERONS. Chapter 18 Campbell 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.

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operons

OPERONS

Chapter 18

Campbell and Reece

e coli synthesizes tryptophan
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 tryptophan1
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 tryptophan2
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
Operator
  • positioned w/in or between promoter & genes
  • controls access of RNA polymerase to genes
operon
Operon
  • includes: promoter, operator, & genes
how does the switch work
How does the “switch” work?
  • just by itself: trpoperon is in “on” position
  • to switch it off: trprepressor 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 trpoperon
      • has own promoter
regulatory genes
Regulatory Genes
  • continuously expressed @ low rate
  • trprepressor is allosteric (2 alternate shapes: active & inactive)
    • trprepressor made in its inactive form with low affinity for trp operator
  • repressor 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 genes1
Regulatory Genes
  • trprepressor is allosteric (2 alternate shapes: active & inactive)
    • trprepressor made in its inactive form with low affinity for trpoperator
    • when tryptophan binds to trp repressor @ allosteric site  protein changes to active form shape  can attach to operator turning operon off
tryptophan
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 animation
trpoperon animation
  • http://highered.mcgraw-hill.com/sites/dl/free/0072835125/126997/animation28.html
2 types of gene regulation
2 Types of (-) Gene Regulation
  • Repressible Operon
    • transcription usually in “on” position but it can be repressed when specific molecule binds to regulatory protein
    • ex: trpoperon
  • Inducible Operon
    • usually in “off”position but can be induced when small molecule interacts with regulatory protein
    • ex: lacoperon
lac operon
Lac operon
  • lactose: 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 operon1
Lac Operon
  • 3 genes:
    • 1: β-galactosidase
    • 2: enzymes that function in lactose utilization
lac operon2
Lac Operon
  • entire 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 lacoperon by binding to operator
  • by itself lac repressor active  binds to operator  switching lacoperon off
repressor
Repressor
  • achieved by specific small molecule called an inducer inactivates repressor
    • lacoperon: 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 lacoperon not being transcribed
inactivation of repressor
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
inducible enzymes
Inducible Enzymes
  • ex: lacoperon
  • their synthesis is induced by a chemical signal (allolactose)
  • generally, function in catabolic pathways
repressible enzymes
Repressible Enzymes
  • ex: tryptophan synthesis
  • generally function in anabolic pathways
  • both repressible & inducible enzymes involves negative control of genes (operons switched off by active form of repressor protein)
slide23

http://highered.mcgraw-hill.com/sites/007337797x/student_view0/chapter13/animation_quiz_-_combination_of_switches__the_lac_operon.htmlhttp://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