Question

How does the lac operon switch off?

Answer 1

The lac or lactose operon is required for the transport and metabolism of lactose in E. coli and some enteric bacterias. Generally, glucose is used as the main carbon source by most bacteria but when it’s not available, lactose is digested and used as an energy source. For this process lac operon is required. When glucose is available in plenty, the lac operon must be repressed to prevent unnecessary digestion of lactose.

Complete answer:
The lac operon is made up of three genes namely lacZ, lacY, and lacA. These genes encode different enzymes. LacZ encodes an enzyme called beta galactosidase, which splits lactose into glucose and galactose. LacY is a permease enzyme that helps to bring lactose into the cell. LacA is a transacetylase and its function in lactose metabolism is not clear. In addition to these genes, the lac operon contains a promoter, operator and CAP-binding site regions.
The transcription of lac operon is inhibited by a protein called lac repressor encoded by the lacI gene. The repressor binds to the operator sequences adjacent to the promoter of the lac operon. Due to repressor binding, the binding of RNA polymerase to the promoter is prevented. And hence, the lac operon will not be transcribed and the metabolism of lactose will be stopped.

Note:
The repressor is activated when there is no lactose and excess glucose is present. Another important property of lacI protein is that it can bind with lactose. When lacI is bound by lactose (that is when lactose is present), the shape of the protein changes and its binding to the operator is prevented. When there is no lactose bound, the lacI binds freely with the operator and gets activated.