Genetics of bacteriophage P22. I. Isolation of prophage deletions which affect immunity to superinfection.

Two kinds of prophage deletions of the Salm.onella phage P22 were isolated. One type of deletion was isolated by simultaneously selecting for loss of a host gene (pro) adjacent to the integrated prophage and for survival at the nonpermissive temperature of a temperature-inducible (tse2) lysogen. The second type of deletion was isolated by transduction to tetracycline resistance using high-frequency-transducing lysates obtained by UV-induction of an unusual tetracycline-resistant (tet ~) P22 lysogen. In the prodeletion strains, a continuous block of genes beginning beyond the left end of the prophage and including all or part of proA is deleted. The c-linked immunity region (immC) is deleted in all the pro-deletion strains. In contrast, in the tet R deletions a continuous block of genes beginning at the right (proC) end of the prophage is deleted. The ran-linked immunity region (immI) is deleted in all of the tet R deletions. These two sets of prophage deletions confirm the established prophage gene order and have been used to compile a comprehensive deletion map of the P22 prophage which includes many previously unmapped genes. The bipartite nature of immunity in P22 is confirmed by the observation that both prodeletions, which laek immC but retain imrnI, and tet~ deletions, which lack immI but retain immC, are each sensitive to superinfection by homoimmune nonvirulent P22 phage.