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, P-R and P-L, transcribing the lysogenic and lytic gene clusters; the two promoter-. proximal genes, cl and mor; and the three CI operator sites, O-R, O-L and O-D. 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 O-R represses transcription from the lysogenic promoter, P-R. Thus, CI regulates its own transcription. Interaction of CI at O-L represses transcription from the lytic promoter, P-L. The presence of only O-L (absence of O-R and O-D) is enough to maintain a bistable system. The distantly located operator site, 01:), functions as a helper site by increasing binding of CI at O-R and O-L. In the immune state, O-D increases repression of the lytic promoter, P-L. Our results strongly support the model that a hexameric form of Cl 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.