Final answer:
The lac operon is regulated by the lac repressor, which inhibits transcription in the absence of lactose, and by catabolite activator protein, which increases transcription when glucose is scarce.
Step-by-step explanation:
The lac operon is regulated through both repression by the lac repressor and activation by the catabolite activator protein (CAP). In the absence of lactose, the lac repressor binds to the operator sequence, preventing transcription of the lac operon genes. When lactose is present, it is converted to allolactose, which then binds to the lac repressor, causing it to release from the operator and allowing transcription.
Further regulation is provided by CAP, which requires cAMP to be active. When glucose is low, cAMP levels rise, and the cAMP-CAP complex forms. This complex can then bind to the promoter region next to the operator, enhancing the affinity of RNA polymerase for the promoter and increasing transcription of the lac operon. Therefore, the presence of lactose allows for de-repression while low glucose conditions activate the operon through cAMP-CAP binding to help the cell efficiently utilize lactose as an alternative energy source.