Repression of Transcription Initiation at 434 PR 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, PRM and PR, determines the phageʹs choice between the lytic and lysogenic development pathways. Here, we demonstrate that 434 repressor bound at 434 OR1 alone is not sufficient to repress transcription from 434 PR, but that 434 repressor bound at 434 OR2 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 λOR1 or λOR2 represses transcription from λPR. The combined results of gel mobility shift and KMnO4 footprinting assays show that while 434 repressor binding to 434 OR2 does not preclude RNA polymerase binding at the PR promoter, it does prevent it from forming open complexes at this promoter. The RNA polymerase-PR 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 OR2 inhibits transcription initiation at the PR promoter by “locking” the RNA polymerase-PR complex into an inactive state instead of “blocking” the access of RNA polymerase to promoter DNA.