Which molecule inactivates the lac repressor by binding to it?

Which molecule inactivates the lac repressor by binding to it?

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

Answer: (D)

Induction of the lac operon happens when an inducer binds to the lac repressor and prevents it from sticking to the operator. Allolactose is the molecule that acts as this inducer by binding directly to the LacI repressor, triggering a conformational change that lowers its affinity for the operator. With the repressor inactivated, RNA polymerase can access the promoter and transcribe the lac operon genes needed to metabolize lactose. Lactose itself is converted into allolactose, so the functional inducer is the allolactose formed inside the cell. Separately, cAMP pairing with CAP boosts transcription when glucose is scarce, helping recruit RNA polymerase, but it does not inactivate the lac repressor. Hence, the molecule that inactivates the lac repressor by binding to it is allolactose.

Induction of the lac operon happens when an inducer binds to the lac repressor and prevents it from sticking to the operator. Allolactose is the molecule that acts as this inducer by binding directly to the LacI repressor, triggering a conformational change that lowers its affinity for the operator. With the repressor inactivated, RNA polymerase can access the promoter and transcribe the lac operon genes needed to metabolize lactose. Lactose itself is converted into allolactose, so the functional inducer is the allolactose formed inside the cell.

Separately, cAMP pairing with CAP boosts transcription when glucose is scarce, helping recruit RNA polymerase, but it does not inactivate the lac repressor. Hence, the molecule that inactivates the lac repressor by binding to it is allolactose.