Superinfection exclusion by P22 prophage in lysogens of Salmonella typhimurium: IV. Genetics and physiology of sieB exclusion

Abstract

The sieB exclusion system of phage P22 interferes with superinfection by several Salmonella phages, but not P22 itself. Vegetative development of the Salmonella phage L proceeds normally in P22 sieB + lysogens until midway through the latent period, when there is a rapid and complete cessation of DNA, RNA, and protein synthesis, resulting in failure to produce any progeny phage. It appears that expression of vegetative functions by the excluded superinfecting phage contributes to the premature arrest of macromolecular synthesis, since superinfecting L phage can establish lysogeny normally in P22 sieB + lysogens. Superinfecting P22 phage escape sieB exclusion by expressing an early phage function ( esc + P22), lacking in L phage. Escape through expression of esc + P22 is dominant in mixed infections. The esc + P22 determinant maps near the P22 c 3 gene and is separable from the P22 immunity regions. Recombination resulting in acquisition of the esc + P22 determinant from the prophage limits the efficiency of exclusion of L phage by P22 sieB + lysogens. The sieB gene of phage P22 is located between erf and gene 24 on the P22 genetic map. This fact poses an interesting problem, since the prophage sieB gene is expressed during superinfection while flanking genes in this region remain repressed. These results indicate that the P22 sieB exclusion mechanism may have elements in common with several other phage exclusion mechanisms, such as those responsible for the exclusion of T4 rII mutants by coliphage λ lysogens and for the exclusion of λ by coliphage P2 lysogens.