Review Question - QID 106639

At a low concentration of glucose, cAMP levels rise resulting in cAMP binding to the catabolite activator protein (CAP). This enhances the activity of RNA polymerase, but since lactose is absent, the repressor is still bound to the lac operator.

The lac operon allows for E. coli to effectively digest lactose and glucose without wasting extra energy when one is absent. In the presence of glucose, cAMP levels are low; therefore, cAMP binding to CAP is reduced. At a low concentration of glucose, increased cAMP levels result in binding to CAP, which enhances the activity of RNA polymerase. In the presence of lactose, a metabolite of lactose called allolactose binds to the repressor protein resulting in a conformational change that removes the repressor from the lac operator. This allows RNA polymerase to transcribe the lac genes. In the absence of lactose, the repressor binds tightly to the lac operator preventing RNA polymerase from transcribing the genes. At a low concentration of glucose and presence of lactose, the repressor is removed from the operator and CAP enhances the activity of RNA polymerase.

Incorrect Answers:
Answer 1 and 4: Although levels of cAMP would be elevated resulting in increased RNA polymerase activity, the repressor protein would still be bound to the DNA due to the absence of lactose. Thus, there would be no transcription of the lac Z, Y, and A genes.
Answer 3: At a low concentration of glucose, cAMP levels would be increased, not decreased.
Answer 5: In the absence of lactose, there would be no allolactose; therefore, the repressor would still be bound to the operator.