Final answer:
Expression of the lac operon in bacteria is repressed when glucose levels are high and is activated when glucose is low and lactose is present, due to complex regulation involving a repressor, CAP, and levels of cAMP.
Step-by-step explanation:
True, expression is low when glucose levels are high in both the lac operon and the galactose operon.
In the case of the lac operon in Escherichia coli, expression is highly regulated based on the availability of glucose and lactose. This operon, responsible for the metabolism of lactose, is activated when glucose levels are low, which triggers an increase in cyclic AMP (cAMP). This compound binds to the catabolite activator protein (CAP), facilitating the binding of CAP to the promoter of the lac operon, thereby promoting transcription. Conversely, when glucose is abundant, the levels of cAMP decrease, leading to reduced CAP binding and a drop in operon expression. Lactose, as an inducer, must also be present for the operon to be expressed, as its metabolite allolactose binds to the repressor protein, preventing it from blocking transcription. Therefore, the lac operon reaches maximal expression in the absence of glucose and presence of lactose. This scenario allows for the economical use of the cell's resources by producing lactose-digesting enzymes only when necessary, ensuring that the primary energy source, glucose, is preferentially used when available.