Which operon is inducible and controls lactose metabolism genes in E. coli?

• A. Trp operon
• B. Gal operon
• C. Lac operon
• D. Ara operon

Answer: (C)

Inducible operons turn on gene expression in response to an inducer. In E. coli, lactose metabolism is governed by the lac operon. Without lactose, the LacI repressor binds the operator and blocks transcription of lacZ, lacY, and lacA. When lactose is present, a derivative called allolactose binds the repressor, releasing it from the operator and allowing RNA polymerase to transcribe the operon. The enzymes produced—beta-galactosidase, lactose permease, and thiogalactoside transacetylase—enable uptake and breakdown of lactose. Regulation is further modulated by glucose via catabolite repression: low glucose raises cAMP, which binds CAP to boost transcription. The other operons manage different nutrients (tryptophan, arabinose, or galactose) and aren’t responsible for lactose metabolism. So the lac operon is the inducible system that controls lactose metabolism genes.