Question

Explain the Lac operon concept.

Answer 1

The lac operon is an inducible system in E. coli that regulates the genes needed for lactose utilisation. It includes a regulator gene, a promoter, an operator and three structural genes: Z, Y and A.

Components

• The i (regulator) gene produces a repressor protein.
• The promoter is the RNA-polymerase binding site, while the operator acts as a switch controlling transcription.
• The structural genes code for enzymes involved in lactose metabolism: Z for β-galactosidase, Y for permease and A for transacetylase.

In the absence of lactose

The active repressor binds to the operator and blocks RNA polymerase from transcribing the structural genes. The lactose-metabolising enzymes are therefore not produced.

In the presence of lactose

Lactose (more precisely, its inducer form allolactose) binds to the repressor and changes its shape. The repressor can no longer bind to the operator. RNA polymerase transcribes the structural genes together into a polycistronic mRNA, and the enzymes needed for lactose metabolism are produced.

Thus, lactose switches the operon on by inactivating its repressor; when lactose is used up, repression can resume.