lac operon control logic: The catabolite activator protein (CAP, also called CRP) exerts what type of control on transcription of lac operon genes?

Introduction / Context: The lac operon integrates two signals: lactose (inducer for LacI) and glucose (through cAMP–CAP). Understanding CAP clarifies how catabolite repression coordinates carbon source preference.

Given Data / Assumptions:

• CAP binds cAMP when glucose is low.
• The CAP–cAMP complex binds a site near the promoter, enhancing RNA polymerase recruitment.
• CAP does not bind the operator as a repressor.

Concept / Approach: CAP functions as a transcriptional activator. Low glucose → high cAMP → CAP–cAMP binds DNA → increases transcription if the lac repressor is not blocking the operator (i.e., if lactose/allolactose is present).

Step-by-Step Solution: Identify CAP’s effector → cAMP. Determine outcome → increased promoter activity. Select option describing positive control.

Verification / Alternative check: Experiments show CAP–cAMP enhances open-complex formation by RNA polymerase at lac and other catabolite-sensitive promoters.

Why Other Options Are Wrong: CAP is not a repressor; its effect is not determined solely by lactose; and it does not regulate β-galactosidase post-translationally.

Common Pitfalls: Confusing CAP’s activation with LacI repression; overlooking the dual-signal logic of the operon.

Final Answer: Positive control (activates transcription when bound with cAMP).