Question

Describe how the Lac repressor and the CAP-cAMP complex regulate the expression of the Lac operon in E. coli.

Answer 1

Lactose metabolism in prokaryotes operates in the form of an operon of 3 structural genes having a common promoter and a common operator. the lac-operon is an inducible operon in which the presence of lactose in the cell induces the expression of the three genes involved in lactose metabolism. In the absence of lactose, the repressor gene (gene i) which is constitutively expressed produces a repressor protein which binds to the operator, thereby blocking the operator site and prevents the binding of RNA polymerase to the promoter site. Hence the transcription of the three structural genes is prevented.

In the presence of lactose, the repressor protein binds with the lactose (making the repressor protein inactive) and the inactive repressor fails to bind the operator region, allowing the RNA polymerase to bind the promoter region and subsequent transcription of three structural genes.

AS far as the CAP-cAMP complex is concerned, it exercises a positive control on the expression of Lac Operon. In presence of lactose, cAMP molecules activates the CAP protein which then allows the RNA polymerase to bind the promoter and brings out the gene expression of structural genes to catabolize lactose.