Which molecule inhibits the lac operon repressor when lactose is present, allowing transcription?

• A. Lactose
• B. Allolactose
• C. cAMP
• D. Glucose

Answer: (B)

Induction of the lac operon hinges on disabling the LacI repressor so transcription can proceed. When lactose is present, some of it is converted to allolactose by beta-galactosidase. Allolactose binds to the LacI repressor, causing a change in shape that decreases LacI’s affinity for the operator. Once LacI releases the operator, RNA polymerase can access the promoter and transcribe the operon.

Allolactose is the actual inducer; lactose itself isn’t the molecule that directly inactivates the repressor. The other factor, cAMP–CAP, boosts transcription when glucose is low, but it doesn’t inactivate the LacI repressor. Glucose presence lowers cAMP levels and reduces CAP activation, which is a separate layer of regulation rather than the direct inactivation of the repressor.