Final answer:
The lac operon is a regulatory system in E. coli that controls lactose metabolism genes. It is activated when glucose levels are low and lactose is present. Cyclic AMP and catabolite activator protein (CAP) play key roles in this activation.
Step-by-step explanation:
The lac operon is a regulatory system found in bacteria, specifically E. coli, that controls the expression of genes involved in the metabolism of lactose. It is activated under two conditions: low or non-existent levels of glucose and the presence of lactose. When glucose levels drop, cyclic AMP (cAMP) accumulates in the cell and binds to an activator protein called catabolite activator protein (CAP), allowing for the expression of the lactose-digesting proteins. This ensures that only when glucose is absent and lactose is present, the lac operon is transcribed.