Final answer:
Catabolite repression associated with the lactose operon in E. coli is best described as an inducible system, as the presence of lactose induces transcription. It is subject to positive control, particularly when glucose levels are low, via cAMP and CAP activation. This system is efficient and saves the cell's energy by only activating in the presence of lactose and absence of glucose.
Step-by-step explanation:
Catabolite repression associated with the lactose operon in E. coli is best characterized by the term inducible system. This is because the lac operon's expression is regulated by the availability of its substrates, such as lactose. When lactose is present, it acts as an inducer that binds to the repressor, preventing the repressor from binding to the operator and allowing for transcription of the lac genes.
Furthermore, the lac operon is subject to both negative and positive control mechanisms. Under positive control, when glucose levels are low, cyclic AMP (cAMP) levels rise and bind to the catabolite activator protein (CAP), which in turn activates transcription of the lac operon. This inducible system ensures that E. coli efficiently uses lactose as an energy source only when necessary, i.e., in the absence of glucose and presence of lactose.
An operon that is transcribed and translated continuously to provide the cell with constant intermediate levels of protein products is termed a constitutive system, which is not the case for the lac operon.