Virulent mutants of phage P22 : II. Physiological analysis of P22 virB-3 and its component mutations

Abstract

The virulent mutant of phage P22, virB-3, consists of two mutations: K5, which maps in the c2 repressor gene, and Vx, which maps between c2 and c3. Although neither P22 K5 nor P22 Vx is virulent, each of these mutants can express gene functions not expressed by other nonvirulent phages in the presence of prophage immunity. In mixed superinfection of a lysogen with P22 virB-3, only a small fraction of the yield consists of c+, c1, or c2 phage even though the virulent grows normally. This is the phenomenon of replication inhibition. In contrast, P22 K5 and to a lesser extent, P22 Vx show escape from replication inhibition. However, neither P22 K5 nor P22 Vx alone replicates extensively in a lysogen. These mutants complement in trans for phage DNA synthesis and phage production. This result indicates that both P22 K5 and P22 Vx express some of the functions required for phage replication in immune conditions. In addition, P22 K5 kills lysogens at low multiplicities, and lysis is observed when these complexes are treated with chloroform.P22 virB-3 is repressed by the c2 gene product made by a c+ phage in mixed infection of a sensitive host. There is an inverse relationship between the burst size and the multiplicity of infection of the c+ phage. P22 virB-3 represses its own growth at high multiplicities even though the K5 mutation maps in the c2 gene and confers a clear plaque phenotype to phage P22. Introduction of a second c2 mutation into the P22 virB-3 genome abolishes this multiplicity effect. These findings demonstrate that P22 virB-3 is sensitive to its own repressor and to that of a coinfecting phage bearing a c2+ allele. We have called this effect multiplicity repression. The residual sensitivity of P22 virB-3 to repressor suggests that at least one of its component mutations is of the operator constitutive type.