How does the lac operon function as an example of negative regulation?

The lac operon exemplifies negative regulation through the action of the lac repressor protein. In the absence of lactose, the lac repressor binds to the operator region of the operon, preventing RNA polymerase from transcribing the genes necessary for the metabolism of lactose. This binding effectively inhibits transcription by blocking the RNA polymerase's access to the promoter.

When lactose is present, it is converted into allolactose, which acts as an inducer by binding to the lac repressor. This binding causes a conformational change in the repressor, leading to its detachment from the operator region. With the repressor removed, RNA polymerase can access the promoter, allowing for the transcription of the downstream genes involved in lactose metabolism.

This mechanism is a classic illustration of negative regulation because the presence of the repressor inhibits the transcription of the operon's genes when lactose is not available, controlling gene expression based on the nutrient availability. The other choices do not accurately describe the mechanism by which the lac operon regulates transcription in response to lactose.