Reciprocal regulation of the early promoter region of bacteriophage HP1 by the Cox and Cl proteins

We have identified a transcriptional switch at the early promoter region of bacteriophage HPI. This switch controls the transcription of the early lytic operon from the P-R1 and P-R2 promoters and the transcription of the lysogenic operon from the P-L promoter. 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 HPI Cox protein repressed the production of P-L transcripts 30-fold, while the HP1 CI protein repressed lytic transcription at least 70-fold. Binding sites for HPI 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. (C) 1997 Academic Press.