Final answer:
The CAP is responsible for positive regulation of the lac operon because CAP bound to the CAP binding site increases the frequency of transcription initiation. When cAMP levels are high due to low glucose, CAP becomes active, binds to the promoter, and facilitates RNA polymerase binding, increasing lac operon transcription.
Step-by-step explanation:
CAP is said to be responsible for positive regulation of the lac operon because c. CAP bound to the CAP binding site increases the frequency of transcription initiation. This process is known as catabolite activation. When extracellular glucose levels are low, cAMP levels increase, binding to CAP and activating it. This CAMP-bound CAP can then bind to the lac operon promoter region. When lactose is available, its metabolite allolactose binds to the lac repressor, preventing it from binding to the operator region. This allows the CAMP-bound CAP to bind to the promoter, bending the DNA and facilitating the binding of RNA polymerase. This event enhances the transcription of the lac operon's genes, which are critical for lactose metabolism.
When glucose levels are high, there is low cAMP, resulting in an inactive CAP, which cannot bind to the promoter region. Therefore, transcription of the lac operon is reduced or aborted as the repressor binds the operator in the absence of allolactose, which would have been converted from lactose if lactose was present. Ultimately, CAP plays a crucial role in switching on genes of the lac operon in response to nutritional cues, allowing E. coli to adapt its metabolism to the availability of sugars in its environment.