Final answer:
In the presence of glucose, cAMP levels are low, which coincides with low expression of the lac operon. For the lac operon to be highly expressed, glucose levels must be low and lactose must be available to remove the lac repressor and allow gene transcription.
Step-by-step explanation:
In the presence of glucose, cAMP levels are low, leading to low expression of the lac operon. The lac operon, an example of an inducible operon, regulates the metabolism of lactose in Escherichia coli. When glucose is available, energy-efficient glycolysis is preferred, and the cells conserve resources by not producing lactose-digesting enzymes. However, when glucose levels are depleted, adenylyl cyclase activity results in increased cAMP, which subsequently binds to the catabolite activator protein (CAP).
The cAMP-CAP complex then binds to the promoter region of the lac operon, enhancing RNA polymerase binding and initiating transcription of genes responsible for lactose metabolism. For maximum activation of the lac operon, two conditions must be met: glucose must be low, prompting cAMP and CAP interaction, and lactose must be present to remove the lac repressor from the operon's operator site, thereby allowing for transcription of the necessary genes.