In lac operon regulation, what is the role of CAP-cAMP?

• A. It represses transcription whenever lactose is present
• B. It directly binds lactose to activate transcription
• C. It activates transcription when glucose is low
• D. It degrades cAMP to shut off transcription

Answer: (C)

CAP-cAMP acts as a positive regulator that boosts lac operon transcription when glucose is scarce. When glucose levels drop, cAMP levels rise; cAMP binds to CAP, and the CAP-cAMP complex binds near the lac promoter. This binding helps RNA polymerase associate with the promoter more effectively, increasing transcription of the lac genes needed to metabolize lactose. This system ensures the operon is most active when a preferred sugar (glucose) is limited. Lactose presence relieves the LacI repressor, but CAP-cAMP’s job is to enhance transcription in low-glucose conditions, not to repress, bind lactose directly, or degrade cAMP.