Regulation of Gene Expression of Bacteria

operon

cluster of genes under the control of one promoter

repressible operon

operon usually on; binding of repressor to the operator shuts down transcription (trp operon)

inducible operon

usually OFF; inducer will inactivate the repressor and turn on transcription (lac operon)

two types of systems, two types of regulation

• positive control: transcription occurs when regulator molecule directly stimulates RNA production

• negative control: gene expression will occur, unless shut off by regulator molecule

lac operon elements

• CAP site

• lacP

• lacO

• lacZ

• lacY

• lacA

• lac terminator

lacP

promoter involved in transcription of lac Z, Y, A

lac Z

encodes for B-galactosidase

lac Y

encodes for lactose permease

lac A

encodes for galactoside transacetylase

2 loci of lac operon control

CAP site and lacO

CAP site

regulatory sequence in DNA recognized by activator protein

operator (lacO)

regulatory sequence in DNA recognized by repressor protein

lacI gene

encodes for repressor protein; NOT part of lac operon

i promoter

promoter of lacI

what type of operon is the lac operon?

inducible

• lactose absent = repressor active = operon OFF

• lactose present = repressor inactive = operon ON

allolactose

side product of B-galactosidase breaking down lactose; small effector molecule that prevents repressor from binding; inducer

lac operon under positive control

• done by CAP (catabolite activating protein) when glucose is low and lactose is present

• when glucose high, CAP can inhibit expression, allowing cell to use glucose

Catabolite activating protein (CAP)

• transcriptional activator

cyclic AMP

synthesized from ATP via adenylyl cyclase; low glucose = high cAMP

cAMP-CAP complex

when cAMP binds to CAP

4 possible environmental conditions for lac operon

1. high glucose, high lactose

2. low glucose, high lactose

3. high glucose, low lactose

4. low glucose, low lactose

high glucose, high lactose

rate of transcription is low

• low cAMP

• high allolactose

• CAP is not binding to CAP site

low glucose, high lactose

transcription of lac operon is high

• CAP binding to CAP site

• repressor not bound

high glucose, low lactose

transcription of lac operon is low (regardless of glucose level)

• repressor is bound to DNA

trp operon

default is ON; regulated by repressor protein

tryptophan

acts as a small effector molecule when concentrations of it are high (feedback inhibition)

trp operon ON

tryptophan absent = repressor inactive

trp operon OFF

tryptophan present = repressor active

anabolic operon

repressible operon; synthesis is repressed at high levels of the end product

catabolic operon

inducible operon; synthesis is induced bt a chemical signal

repressible and inducible operons

BOTH are negative, but default state of repressor is different