MCAT Bio/Biochem practice questionGene regulation in bacteria

The answer

Correct answer: A. The lac operon would be maximally expressed because the repressor is inactivated by allolactose and CAP is bound to the promoter.

Tested concept: The lac and trp operons

Explanation

For maximal expression of the lac operon, two conditions must be met: lactose must be present, and glucose must be absent or at very low levels. The presence of lactose leads to the production of allolactose, which binds to the lac repressor, causing it to detach from the operator. This allows RNA polymerase to transcribe the operon. Simultaneously, very low glucose levels lead to high intracellular cAMP, which binds to the Catabolite Activator Protein (CAP). The cAMP-CAP complex then binds to the CAP binding site near the promoter, enhancing RNA polymerase's affinity for the promoter and significantly increasing transcription. Therefore, with high lactose and very low glucose, the lac operon is maximally expressed. The claim that the operon would be completely repressed due to even a small amount of glucose is incorrect because low glucose levels allow CAP activation, which promotes expression. The claim that the operon would be minimally active because glucose is always the primary energy source is incorrect because while glucose is preferred, its very low concentration removes the catabolite repression effect. The claim that the operon would be partially induced because the repressor maintains binding affinity is incorrect because allolactose binding effectively inactivates the repressor, causing it to fully dissociate from the operator.