Gene Regulation

Operon Structure

a group of genes that are regulated as a single unit

Operator

repressors bind here & prevent transcription

Types of Gene Regulation in Prokaryotes

• Constitutive

• Induction

• Activation

• Repression

• Attenuation

Constitutive

no regulation → gene always expressed

Induction

• repressor blocks operator basal transcription

• Inducer causes repressor to fall off operator → transcription begins

Repression

• repressor can’t bind without corepressor → transcription occurs

• corepressor binds to repressor which binds to operator → very low transcription

Activation

• -35 & -10 not recognized well by RNA pol

• Activator can’t bind to promoter w/o coactivator → weak transcription

• coactivator binds to activator which binds to promoter → RNA pol binds well - good transcription

lac Operon

a group of bacterial genes that controls the breakdown of lactose; it is turned on when lactose is present and glucose is low

• is regulated by induction & activation

lac Operon Induction

• Repressor blocks operator

• lactose (inducer) causes repressor to fall off operator → transcription occurs

Diauxic Growth

cells will use glucose before they use lactose because it takes more energy

Catabolite Repression (lac Operon activation)

• bacterium will utilize most readily catabolizable carbon source first (glucose) before using other carbon sources

• glucose levels also regulate expression of the lac operon

• when glucose is present, the lac operon is repressed (catabolite repression) even if lactose is present

Attenuation

• premature termination of transcription

• b/c RNA is single stranded, often stem loop structures can form

• some stem loop structures are rho independent (3&4) transcriptional terminators which causes transcription to stop

trp Operon

a group of bacterial genes that controls the production of enzymes needed to synthesize the amino acid tryptophan; is turned off when tryptophan is present in high amounts and turned on when tryptophan is low

• regulated by repression and attenuation

Repressor

a regulatory protein that binds to DNA and blocks transcription of a gene or operon, preventing gene expression

Inducer

a molecule that binds to a repressor or regulatory protein and activates gene expression by allowing transcription to occur

Activator

a regulatory protein that increases gene transcription by helping RNA polymerase bind to DNA or by enhancing its activity

Coactivator

a protein that helps activator proteins increase gene transcription by assisting RNA polymerase function

• binds to activator

Corepressor

a small molecule that binds to a repressor protein and activates it, allowing the repressor to bind DNA and block transcription

β-galactosidase

• breaks down lactose

• made by lacZ

lactose permease

• transports lactose into cell

• made by lacY

Adenylate cyclase

makes cAMP

• is regulated by glucose

cAMP

• A small signaling molecule

• Made by the enzyme adenylate cyclase

• binds to CAP and activates it

CAP

• A protein (activator)

• Binds to DNA at the CAP site (upstream of promoter)

• Cannot bind DNA unless cAMP is attached

Function: helps RNA polymerase bind → increases transcription