The tripartite immunity system of phages P1 and P7.

Prophages P1 and P7 exist as unit copy DNA plasmids in the bacterial cell. Maintenance of the prophage state requires the continuous expression of two repressors: (i) C1 is a protein which negatively regulates the expression of lytic genes including the C1 inactivator gene coi, and (ii) C4 is an antisense RNA which specifically inhibits the synthesis of an anti-repressor Ant. In addition, C1 repression is strengthened by lxc encoding an auxiliary repressor protein. The repressors C1, C4 and Lxc are components of a tripartite immunity system of the two phages. Here, the mode of action of these regulatory components including their antagonists Coi and Ant is described.