Ace the 2026 A2 Genetics Exam – Master Protein Control & Gene Expression Like a Pro!

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

cAMP-CRP complex binds the promoter and facilitates RNA polymerase recruitment.

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.

Repressor binds the operator to block transcription.

RNA polymerase cannot bind the promoter without activator.

Inducer binds the repressor to relieve repression.