Gene Regulation Prokaryotes

lac operon

consists of

lac I - produces lac I repressor protein that binds to operator

lacZ - encodes beta galactosidase

lacY - encodes galatosidase permase (breakdowns lactose)

lacA

Positive Regulation

low levels of glucose

• cAMP binds to CAP which binds to CAP binding site to recruit RNA polymersase to begin transcribing

Negative Regulation

absence of lactose

• repressor protein binds to operator, preventing RNA polymerase from binding to promotor and transcribing

presence of glucose

• inhibits import of lactose

• galactosdie permease is inhibited

cis acting and transacting elements

cis acting - on the same dna molecule (promotor, operator)

transacting - made elsewhere (lac I repressor protein)

lac I repressor protein

when no lactose is present, it binds to the operator, preventing RNA polymerase from binding to the promotor to transcribe

CAP Protein

positive regulation

• cAMP binds to CAP which binds to CAP binding site to recruit RNA polymerase

inducer

disables the repressor protein

• lactose binds to repressor protein

• changes confmation of DNA at binding site

• RNA polymerase can bind at promotor and transcribe

given mutations in:

lacI - a repressor protein would not be made to bind to the operator, so the operon is always on

lacI repressor protein - stay bound to the operator so operon is always on

lacZ - B galactosidase can’t break down lactose

lacY - galactosidase permease can’t transport lactose

constitutive metabolism mutant

lacI - would not create repressor protein that bind to operator and prevent RNA polymerase from transcribing, so operon is always on

IPTG

artificial inducer that binds to repressor (acts like lactose)

• IPTG cant be broken down by galatosidase

used experimentally:

synthesis of proteins - gene of interest is cloned

identifying gene regulatory elements

• attach lacZ to different DNA sequences

• add IPTG to turn operon on and observe if lacZ is expressed