Final answer:
In a negatively controlled operon like the lac operon, transcription is enabled when a repressor is prevented from binding to the operator, as occurs when lactose is present and glucose is absent in the cell.
Step-by-step explanation:
For an operon under negative control to be transcribed, a repressor protein that normally binds to the operator, preventing transcription, must be prevented from binding. In the case of the lac operon, which is one such operon under negative control, transcription occurs when lactose is present and glucose is absent. Lactose metabolite, allolactose, binds to the lac repressor and changes its conformation so that the repressor cannot bind to the operator.
Without the repressor in place, RNA polymerase can bind to the promoter and start transcription of the genes necessary for lactose metabolism. This regulation ensures that the cell conserves energy by not producing enzymes for lactose metabolism unless lactose is available and glucose, a preferable energy source, is not.