Final answer:
The lac operon is under negative repression, fully expressed when lactose is present and glucose levels are low. This operon exhibits precise regulation via the repressor protein and catabolite activator protein to ensure efficient energy usage in E. coli.
Step-by-step explanation:
The lac operon is also under negative repression, expressed at highest levels when lactose is present and glucose concentration is low.
The lac operon is an excellent example of gene regulation in prokaryotes. When glucose levels are sufficient, a repressor protein binds to the operator, preventing transcription of the lac operon. In E. coli, lactose is used as an alternative energy source only when glucose is not available. When glucose levels drop, the repressor is removed in a process called derepression. Additionally, the presence of lactose leads to the production of allolactose, which in turn acts as an inducer by binding to the repressor and preventing its binding to the operator. This allows RNA polymerase to transcribe the genes necessary for lactose metabolism.
Furthermore, the lac operon is subject to catabolite activation. Catabolite activator protein (CAP), when bound to cAMP (due to low glucose), enhances the transcription of the lac operon. Together, these systems ensure that the lac operon is most active only when lactose is available, and glucose, the preferred energy source, is scarce.