Final answer:
The ability of glucose to repress the lac operon depends on effector molecules such as glucose CAP and the repressor protein. When glucose is present as a preferred carbon source, the repressor protein binds to the operator site, blocking transcription of the lac operon.
Step-by-step explanation:
The ability of glucose to repress the lac operon depends on the effector molecules glucose CAP and the repressor protein. The lac operon in E. coli is usually repressed when glucose is available as a preferred carbon source. The repressor protein binds to the operator site, blocking transcription of the lac operon.
In the presence of lactose, the allolactose, an isomer of lactose, acts as a positive modulator or inducer for the lac operon by binding to the repressor protein and releasing it from the operator site. Additionally, glucose levels affect the lac operon through the cAMP-CAP complex, where low glucose levels result in high levels of cAMP, which binds to catabolite activator protein (CAP) and activates transcription.