Negative regulation of the lac operon by the Lac repressor. In the absence of lactose (top), the Lac repressor binds the operator region and thus prevents RNA polymerase from leaving the promoter site and transcribing the operon. In the presence of lactose (bottom), its metabolite allolactose binds the Lac repressor, resulting in a conformational change that causes the repressor to dissociate from the operator. RNA polymerase can then initiate transcription. (Note that allolactose is enlarged for clarity; it is actually much smaller relative to the Lac repressor.)