Which molecule acts as the inducer for the lac operon by inactivating the lac repressor?

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

Answer: (C)

The induction of the lac operon happens when a molecule binds to the lac repressor and causes it to release the operator, allowing transcription to proceed. The inducer for this system is allolactose, which is formed from lactose by the enzyme beta-galactosidase. When allolactose binds the repressor, the repressor changes shape and can no longer tightly bind the operator, lifting the block and enabling RNA polymerase to transcribe the lac genes.

cAMP's role is different: it binds to CAP to form a complex that helps recruit RNA polymerase and boost transcription when glucose is scarce. This is a separate regulatory mechanism (catabolite activation) and does not inactivate the lac repressor. Glucose presence lowers cAMP levels, reducing CAP activation, which dampens transcription, but that’s not the inducer for the repressor.

So the molecule that directly inactivates the lac repressor to start transcription is allolactose.