In lac operon regulation, which molecule inactivates the lac repressor when lactose is present?

• A. Allolactose activates the repressor
• B. Allolactose inactivates the lac repressor when lactose is present
• C. Allolactose binds CAP to activate transcription
• D. Allolactose has no role in lac operon regulation

Answer: (B)

In lac operon regulation, allolactose is the inducer that relieves repression. It is produced from lactose by the enzyme beta-galactosidase and binds to the LacI repressor. This binding causes a conformational change in LacI that lowers its affinity for the operator, so the repressor releases from the DNA. With the repressor no longer blocking the operator, RNA polymerase can access the promoter and transcribe the operon’s genes (lacZ, lacY, lacA).

CAP's role is separate and is controlled by cAMP levels: when glucose is scarce, cAMP binds CAP to help recruit RNA polymerase, but allolactose does not activate CAP. So allolactose inactivates the lac repressor in the presence of lactose, enabling transcription.