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OPERONS

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

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  1. OPERONS Chapter 18 Campbell and Reece

  2. 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

  3. 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

  4. 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

  5. Operator • positioned w/in or between promoter & genes • controls access of RNA polymerase to genes

  6. Operon • includes: promoter, operator, & genes

  7. 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

  8. 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

  9. 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

  10. 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

  11. trpoperon animation • http://highered.mcgraw-hill.com/sites/dl/free/0072835125/126997/animation28.html

  12. 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

  13. 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

  14. Lac Operon • 3 genes: • 1: β-galactosidase • 2: enzymes that function in lactose utilization

  15. 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

  16. 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

  17. 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

  18. Lac Repressor

  19. Inducible Enzymes • ex: lacoperon • their synthesis is induced by a chemical signal (allolactose) • generally, function in catabolic pathways

  20. 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)

  21. 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

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