Final answer:
Binding of the cAMP-CAP complex to the CAP site causes DNA bending, enhancing RNA polymerase binding and transcription of the lac operon, especially when glucose is low and lactose is present.
Step-by-step explanation:
When the cAMP-CAP complex binds the CAP site near the lac promoter, it causes the DNA in the promoter-operator region to bend. This bending of the DNA facilitates the binding of RNA polymerase to the promoter, ultimately enhancing the transcription of the lac operon genes. The presence of cAMP-bound CAP is essential for this process because it induces a structural change in the DNA that allows better access for RNA polymerase. Moreover, under conditions where glucose is scarce and lactose is present, the lac operon's transcription is maximally activated. This results in the production of enzymes such as β-galactosidase, which hydrolyzes lactose into glucose and galactose, allowing the cell to utilize lactose as an alternative energy source.