Degradation of the disaccharide lactose into the monosaccharides glucose and galactose requires multiple proteins, which are encoded by the lacZ, lacY, and lacA genes of the lac operon. Through the organization of these genes into an operon under the control of a single promoter (P), transcription of all of these genes can be regulated together in prokaryotes.
The Lac I repressor is encoded in a separate operon adjacent to the lac operon. When the Lac I repressor binds to the lac operon's operator (O) sequence, which lies between the promoter and the lac Z gene, it prevents transcription of the lac genes because the progression of RNA polymerase through the operator region is blocked. However, the Lac I repressor is inactivated in the presence of lactose.
Additionally, in order to minimize transcription of the lac operon when other more preferred carbon sources, like glucose, are present, maximal transcription of the lac operon also requires activation. Activation occurs when glucose levels are depleted, causing a rise in cellular levels of cyclic AMP (cAMP).
When this occurs, cAMP binds to the catabolite activator protein (CAP), and this complex binds to a CAP-binding site near the lac operon's promoter. CAP-cAMP binding to this CAP-binding site facilitates RNA polymerase binding to the promoter, thus activating transcription. This mechanism of transcriptional control is called catabolite repression.
Which of the following BEST describes conditions allowing for maximal transcription of the lac operon genes?