Figure 1. The genetic switch region of lambda phages (lambda, 434, P22). The repressor and cro genes lie on opposite sides of the operator region, containing three operators (OR1, OR2, OR3). The two genes are transcribed in opposite directions from their respective promoters, which overlap in the middle operator, OR2.

(Top) The situation in a lysogenic cycle. Lambda repressor throws the genetic switch to repressor transcription and cro gene silencing. The lambda repressor protein binds OR2 and OR1, preventing RNA Polymerase from transcribing the cro gene, and promoting repressor transcription from the repressor maintenance promoter. Unlike cro, the repressor has an activation domain that promotes RNA Polymerase binding to the repressor promoter.

(Bottom) The situation in a lytic infection. Cro protein occupies OR3, preventing RNA Polymerase from initiating transcription from the repressor promoter. RNA Polymerase transcribes the cro gene (and other genes), producing more of cro protein, which silences repressor transcription.