Which regulatory mechanism increases lac operon transcription when glucose is scarce?

• A. cAMP-CRP complex binds the promoter and facilitates RNA polymerase recruitment.
• B. Repressor binds the operator to block transcription.
• C. RNA polymerase cannot bind the promoter without activator.
• D. Inducer binds the repressor to relieve repression.

Answer: (A)

When glucose is scarce, cAMP levels rise and form a complex with CRP (CAP). This cAMP-CRP complex binds near the lac promoter and helps RNA polymerase bind and initiate transcription, boosting lac operon expression to use available lactose. This positive regulation mechanism—the CAP-cAMP activator—is why transcription increases under low-glucose conditions. Repression by the LacI protein would lower transcription, not raise it; RNA polymerase can bind with some activity on its own, but the activator greatly enhances transcription; and derepression by an inducer binding to the repressor depends on lactose presence, not specifically on low glucose.