Which statement correctly describes the lac operon and its regulation?

• A. Lac operon is regulated by a LacI repressor only, with no activator
• B. Lac operon is regulated by CAP activator only, with no repressor
• C. The lac operon is regulated by LacI repressor and CAP activator in response to lactose and glucose levels
• D. The lac operon regulation does not respond to lactose or glucose

Answer: (C)

Regulation of the lac operon integrates two signals: lactose availability and glucose level. A LacI repressor binds to the operator to block transcription. When lactose is present, it is converted to allolactose, which binds LacI and releases it, allowing transcription to start. However, transcription is further regulated by CAP (activator) that binds near the promoter only when cAMP levels are high, which happens when glucose is scarce. CAP binding helps RNA polymerase bind and increases transcription. Therefore, full expression occurs when lactose is present to remove repression and glucose is low to activate CAP. If lactose is absent, the repressor stays bound and the operon remains off regardless of CAP. If glucose is high, CAP is inactive, so expression is weak even if lactose is present. This combination explains why the lac operon responds to both lactose and glucose levels.