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.