The Role of MOR and the CI Operator Sites on the Genetic Switch of the Temperate Bacteriophage TP901-1

A genetic switch controls whether the temperate bacteriophage TP901-1 will enter a lytic or a lysogenic life cycle after infection of its host, Lactococcus lactis. We studied this bistable switch encoded in a small DNA fragment of 979 bp by fusing it to a reporter gene on a low-copy-number plasmid. The cloned DNA fragment contained the two divergently oriented promoters, PR and PL, transcribing the lysogenic and lytic gene clusters; the two promoter-proximal genes, cI and mor; and the three CI operator sites, OR, OL and OD. We show that mor encodes a protein and that this protein in concert with CI is required for the bistability. Furthermore, interaction of CI at OR represses transcription from the lysogenic promoter, PR. Thus, CI regulates its own transcription. Interaction of CI at OL represses transcription from the lytic promoter, PL. The presence of only OL (absence of OR and OD) is enough to maintain a bistable system. The distantly located operator site, OD, functions as a helper site by increasing binding of CI at OR and OL. In the immune state, OD increases repression of the lytic promoter, PL. Our results strongly support the model that a hexameric form of CI binds cooperatively to the three operator sites in the immune state forming a CI–DNA loop structure. Finally, we show that in the anti-immune state, repression of the lysogenic promoter is independent of the known CI operator sites but requires the presence of both CI and MOR.