Superinfection exclusion by P22 prophage in lysogens of Salmonella typhimurium. II. Genetic evidence for two exclusion systems.

Mutants of the Salmonella phage P22 which, as prophages, do not prevent the growth of superinfecting virulent P22 phage were isolated. These mutants, called sieA-, retain some ability to exclude the heteroimmune phages L and MG178. Prophages carrying mutations at another locus, sieB, lose entirely the ability to exclude the heteroimmune phages. The sicmutants of Rao (1968) were shown to be sieA-sieBdouble mutants. The sieA and sieB loci are located on the P22 genetic map near the imm1 and immC regions. The exclusion properties of sieA and sieB alleles are independent of the immunity specificity of the prophage. The A exclusion system (defined by sieAmutants) appears to be nonspecific and excludes phages P22, MG178, or L. The A exclusion system is entirely responsible for the exclusion of generalized transducing particles by P22 lysogens. The B system, on the other hand, acts only on the heteroimmune phages. The two exclusion systems appear to act independently.