Final answer:
Beta-galactosidase production is regulated at the transcriptional level through the lac operon in E. coli, being activated when lactose is available and glucose is not.
Step-by-step explanation:
When glucose is no longer available as a nutrient source for bacteria, and lactose becomes the predominant nutrient source, beta-galactosidase production is regulated at the transcriptional level. This is seen in the operation of the lac operon in E. coli. The gene encoding beta-galactosidase (lacZ) is part of an operon that is normally repressed when glucose is present. In the absence of glucose, and when lactose is available, the lac repressor is inactivated by allolactose. This allows RNA polymerase to transcribe the lac operon, leading to the production of beta-galactosidase, which is necessary for lactose utilization.
Additionally, the presence of cAMP levels increase when glucose is absent, which binds to the catabolite activator protein (CAP). The cAMP-CAP complex then binds to the lac operon, thereby enhancing transcription. This two-tier regulation ensures that the lac operon, and therefore beta-galactosidase, is expressed only when required, which is when lactose is present and glucose is not.