Inducible and repressible operons

Bacteria and their environment

bacteria must respond quickly to changes in their environment. one of the main ways it does this is by regulating gene expression

What is gene regulation

Gene regulation allows the cell to turn off and on genes depending on whether or not they are needed.

operon model and operons

the operon model is the system for describing bacterial gene regulation. An operon is a group of genes transcribed together as one mRNA under the control of a single promoter operator. Operons allow bacteria to coordinate expression of genes in the same pathway.

inducible operons

usually involved in catabolic pathways

normally OFF but can be turned ON when the substrate is present

controlled by inactivating a repressor when and inducer binds to it

Repressive operons

usually involved in biosynthetic pathways

normally on but can be turned off when end product is abundant

Controlled by activating repressor when a corepressor binds to it

Structure of an operon

Inducible and repressible act different but their structure is similar. Promoter: where RNA polymerase binds. Operator: binding site for regulatory protein and structural genes: encode proteins needed for the pathway. Some operons also have binding sites, such as the UP site for CAP in the lac operon. Transcription depends on whether regulatory proteins block or assist RNA polymerase

inducible operons: the Lac operon

The lac operon of E.coli is an inducible operon. it allows the cell to metabolise lactose only when glucose is scarce and lactose is available.

Lac operon is OFF when..

when lactose is absent. the lacl repressor is active and it binds to the operator and blocks RNA polymerase. A DNA loop forms that prevents transcription

Lac operon ON when

when lactose is present. A lactose derivative binds the repressor. this causes a conformational change. the repressor cannot bind to the operator. RNA polymerase now transcribes the operon

Positive control by CAP-cAMP

the lac promoter is weak. Maximum transcription requires CAP. when glucose is low cAMP levels rise. cAMP binds to the UP site. this impoves RNA polymerase binding. this ensures glucose is used first which is more efficient

Repressible operons the trp operon

the trp operon contains genes required for tryptophan biosynthesis, Because tryptophan is essential the operon is normally ON

trp operon ON

when tryptophan is scarce. the trp repressor is inactive on its own. it cannot bind the operator. rna polymerase can freely transcribe the operon. cells need to make tryptophan so transcription proceeds.

trp operon OFF

when tryptophan is abundant. Tryptophan acts as a corepressor. It binds the trp repressor and activates it. The active repressor binds the operator and blocks transcription. This prevents overproduction of tryptophan, which saves energy