It decreases the levels of cAMP in the cell, repressing transcription from the lac operon.
Explanation:
When glucose is absent, cAMP serves as coactivator binds to CRP, the catabolite gene activator protein. The CRP-cAMP complex binds to the site near the lac promoter and stimulates the expression of the operon by RNA polymerase many folds.
Catabolite repression refers to inhibition of the synthesis of enzymes of lactose catabolism when glucose is present as an energy source. In the presence of glucose, synthesis of cAMP is inhibited resulting in its lower cellular concentration. The lower cAMP levels do not allow the binding of cAMP and CRP. The result is reduced expressed of lac operon.
