Site c27 in phage P22 and control of the pathway to lysogeny.

Phage P22 mutation c27 defines a site required for establishment , but not maintenance of repressor synthesis. This study confirms that P22 c27 is able to synthesize repressor if active repressor is present. An interaction involving gene products of c1 and c3 and the site c27 retards expression of the lytic genes of P22. Mutations in gene c1 eliminate the retardation of lytic gene expression, but c27 does not alleviate the retardation. These results are used to construct a model that postulates that binding of c1 and c3 products to DNA at or near c27 is sufficient to cause retardation of lytic gene expression. The functioning of c27 is contrasted to that of the analogous cy mutants of lambda. The effect of the c27 mutation upon alleviation of "cl repression" was studied in a partial revertant of Salmonella typhimurium Pox-1 in which c1 repression is exaggerated. The higher frequency of lysogenization seen in the mutant host is related to enhanced cl repression.