Reciprocal Regulation of the Early Promoter Region of Bacteriophage HP1 by the Cox and CI Proteins

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We have identified a transcriptional switch at the early promoter region of bacteriophage HP1. This switch controls the transcription of the early lytic operon from the PR1and PR2promoters and the transcription of the lysogenic operon from the PLpromoter. The start sites of the three promoters were mapped, and using a chloramphenicol acetyl transferase assay, we have investigated the levels of transcription from the promoters in the absence or in the presence of two phage-encoded transcription factors: HP1 Cox and HP1 CI. The HP1 Cox protein repressed the production of PLtranscripts 30-fold, while the HP1 CI protein repressed lytic transcription at least 70-fold. Binding sites for HP1 Cox and CI were identified in the early promoter region; mutations of these sites eliminated transcriptional repression. In addition, a mutant CI protein was isolated which is temperature sensitive for repression. Taken together, these data demonstrate the reciprocal regulation of a transcriptional switch in which the actions of the two phage-encoded proteins at the phage early promoters determine the choice between lytic and lysogenic growth.