Superinfection immunity and prophage repression in phage P1 and P7 III. Induction by virulent mutants

Abstract

Two types of virulent mutants ( virB and virC) have been isolated for phage Pl; both induce a Pl prophage to replicate after superinfection of a lysogen. The genetic lesion of P1 virB (and P7 virB) is located in the immunity-specific region of the phage, and these mutants induce only a homoimmune prophage. The virB mutants appear to depend for prophage induction on titration of the phage-specific c4 immunity repressor in the lysogen, since DNA synthesis of the superinfecting phage is needed for this induction process. The P1 virC mutation, which is located at the left end of the phage map, induces both homoimmune and heteroimmune prophages. The virC mutant causes prophage induction in the absence of phage DNA replication and may induce because of constitutive synthesis of a vegetative repressor ( c1) antagonist.