Bacteriophage P1 gene 10 is expressed from a promoter-operator sequence controlled by C1 and Bof proteins.

Gene 10 of bacteriophage P1 encodes a regulatory function required for the activation of P1 late promoter sequences. In this report cis and trans regulatory functions involved in the transcriptional control of gene 10 are identified. Plasmid-borne fusions of gene 10 to the indicator gene lacZ were constructed to monitor expression from the gene 10 promoter. Production of gp10-LacZ fusion protein became measurable at about 15 min after prophage induction, whereas no expression was observed during lysogenic growth. The activity of an Escherichia coli-like promoter, Pr94, upstream of gene 10, was confirmed by mapping the initiation site of transcription in primer extension reactions. Two phage-encoded proteins cooperate in the trans regulation of transcription from Pr94: C1 repressor and Bof modulator. Both proteins are necessary for complete repression of gene 10 expression during lysogeny. Under conditions that did not ensure repression by C1 and Bof, the expression of gp10-LacZ fusion proteins from Pr94 interfered with transformation efficiency and cell viability. Results of in vitro DNA-binding studies confirmed that C1 binds specifically to an operator sequence, Op94, which overlaps the -35 region of Pr94. Although Bof alone does not bind to DNA, together with C1 it increases the efficiency of the repressor-operator interaction. These results are in line with the idea that gp10 plays the role of mediator between early and late gene transcription during lytic growth of bacteriophage P1.