Which statement best describes how lactose and glucose regulate the lac operon in E. coli, including the roles of the lac repressor and CAP-CAMP?

Which statement best describes how lactose and glucose regulate the lac operon in E. coli, including the roles of the lac repressor and CAP-CAMP?

• A. Lactose activates the lac repressor preventing transcription.
• B. When glucose is high, cAMP levels rise and CAP-CAMP enhances transcription.
• C. Lactose inactivates the lac repressor allowing transcription; when glucose is low, cAMP rises and CAP-CAMP enhances RNA polymerase binding, increasing transcription.
• D. Lactose has no effect on the lac operon; glucose controls all transcription independently.

Answer: (C)

Lactose relief of repression plus glucose-dependent activation. Lactose (via its metabolite allolactose) binds to the lac repressor and prevents it from sticking to the operator, so RNA polymerase can access the promoter and start transcription. At the same time, CAP binds cAMP to form CAP-CAMP, and this complex activates transcription by helping RNA polymerase bind more effectively to the promoter. cAMP levels rise when glucose is scarce, so CAP-CAMP is active under low glucose conditions. Thus, when lactose is present and glucose is low, both relief of repression and CAP-CAMP–mediated activation work together for strong transcription. If lactose is absent, the repressor stays bound and blocks transcription despite CAP-CAMP. If glucose is high, cAMP is low and CAP-CAMP isn’t activating transcription, even if lactose is present. The described statement captures both mechanisms, making it the best choice.

Lactose relief of repression plus glucose-dependent activation. Lactose (via its metabolite allolactose) binds to the lac repressor and prevents it from sticking to the operator, so RNA polymerase can access the promoter and start transcription. At the same time, CAP binds cAMP to form CAP-CAMP, and this complex activates transcription by helping RNA polymerase bind more effectively to the promoter. cAMP levels rise when glucose is scarce, so CAP-CAMP is active under low glucose conditions.

Thus, when lactose is present and glucose is low, both relief of repression and CAP-CAMP–mediated activation work together for strong transcription. If lactose is absent, the repressor stays bound and blocks transcription despite CAP-CAMP. If glucose is high, cAMP is low and CAP-CAMP isn’t activating transcription, even if lactose is present. The described statement captures both mechanisms, making it the best choice.