Repression of transcription initiation at 434 P_R by 434 repressor: Effects on transition of a closed to an open promoter complex

The lambdoid bacteriophage repressors function both as transcription activators and repressors. Regulation of transcription at the adjacent, but divergent promoters, P_RM and P_R, determines the phage's choice between the lytic and lysogenic development pathways. Here, we demonstrate that 434 repressor bound at 434 O_R1 alone is not sufficient to repress transcription from 434 P_R, but that 434 repressor bound at 434 O_R2 alone is necessary and sufficient to repress PR transcription. This is different from what occurs in the related bacteriophage λ, in which binding of λ repressor to either λO_R1 or λO_R2 represses transcription from λP_R. The combined results of gel mobility shift and KMnO₄ footprinting assays show that while 434 repressor binding to 434 O_R2 does not preclude RNA polymerase binding at the P_R promoter, it does prevent it from forming open complexes at this promoter. The RNA polymerase-P_R complexes that form in the presence of repressor are heparin-resistant and the DNA is not melted. This observation indicates that 434 repressor bound at 434 O_R2 inhibits transcription initiation at the P_R promoter by "locking" the RNA polymerase-P_R complex into an inactive state instead of "blocking" the access of RNA polymerase to promoter DNA.