Chapter 18 Campbell and Reece 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 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 & genes controls access of RNA polymerase to genes

Operon includes: promoter, operator, & genes

How does the “switch” work? just by itself: trp operon is in “on” position to 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 Genes continuously expressed @ low rate trp repressor is allosteric (2 alternate shapes: active & inactive) trp repressor 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 Genes trp 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 animation http://highered.mcgraw- hill.com/sites/dl/free/0072835125/126997/ animation28.html

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: trp operon Inducible Operon usually in “off”position but can be induced when small molecule interacts with regulatory protein ex: 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 Operon 3 genes: 1: β-galactosidase 2: enzymes that function in lactose utilization

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 lac operon by binding to operator by itself lac repressor active  binds to operator  switching lac operon off

Repressor achieved 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 Enzymes ex: lac operon their synthesis is induced by a chemical signal (allolactose) generally, function in catabolic pathways

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)

http://highered. mcgraw- hill http://highered.mcgraw- hill.com/sites/007337797x/student_view0/ chapter13/animation_quiz_- _combination_of_switches__the_lac_ope ron.html http://www.wwnorton.com/college/biolog y/mbio/animations/main.asp?chno=ch10a0 1