Simplified representation of the lac operon. When the Lac repressor is not bound to the operon’s promoter region, RNA polymerase binds to the promoter and transcribes several linked genes, here labeled A, B, and C. The Lac repressor binds to two operators on either side of the promoter to shut down transcription, apparently forming a loop in the DNA that prevents RNA polymerase from binding to the promoter. Each operator consists of an inverted repeat (top inset). The helix- n- n-