A phage P22 gene controlling integration of prophage.

A mutually noncomplementing group of phage P22 mutants (L mutants) have been isolated which cannot accomplish a late step necessary for prophage integration. L mutants produce turbid plaques and lead to cell survival on high multiplicity infection, but the infecting phage genomes are incapable of integrating as prophage and are progressively diluted among the segregating cells. L mutants can become prophage by complementation with L+ phage. The L+ gene apparently produces a diffusible product which acts lrans to accomplish integration. Experiments with temperature sensitive L mutants show that the gene begins acting late, after the infected cells have recovered and begun to divide, and does not accomplish its effect in all the cells until at least the third generation. L lysogens formed by complementation are stable. The L+ gene is thus necessary for integration but not for maintenance of the lysogenic state.