operons chapter 18 campbell and reece. e. coli synthesizes tryptophan 5 genes clustered together...

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