Transcription of phage genes cro and cl

(a) Promoters PR and PRM overlap three operator sites—OR1, OR2, and OR3—that are competitively bound by regulatory proteins. (b) The cro gene is transcribed from PR. Cro protein binds OR3 and OR2, leading to transcription of genes that generate the lytic cycle. (c) The cl gene is transcribed from PRM to produce repressor that binds to OR1 and drives additional cl transcription. Other gene transcription is blocked, and lysogeny is established(a) Promoters PR and PRM overlap three operator sites—OR1, OR2, and OR3—that are competitively bound by regulatory proteins. (b) The cro gene is transcribed from PR. Cro protein binds OR3 and OR2, leading to transcription of genes that generate the lytic cycle. (c) The cl gene is transcribed from PRM to produce repressor that binds to OR1 and drives additional cl transcription. Other gene transcription is blocked, and lysogeny is established

cl P _O _P RM RR CFO F—+-—-+—-! 17 bp 17 bp 17 bp transcript transcript (b) RNA polymerase Cro protein (C) RNA polymerase A repressor