Western blot (immunoblot) analysis of cell extracts from induced bacteriophage lambda lysogens probed with S-protein-specific antibody (raised against an S-,I-galactosidase fusion protein) demonstrated that the bacteriophage lambda S protein begins to appear 10 min after phage induction and is localized to the inner membrane at all times during the lytic cycle. Between 100 and 1,000 molecules of S protein per cell were present at the time of phage-induced lysis. Western blots of chemically cross-linked membranes from induced lysogens showed a ladder of bands at 18, 24, 32, and 42 kilodaltons (the S-protein monomer ran at 8 kilodaltons) that reacted with anti-S-protein antibody. Thus, the S protein appears to reside in the inner membrane as a multimer, and the molecular weights of the cross-linked species are consistent with those of S-protein homopolymers. Sodium dodecyl sulfate-resistant dimers were also detected when S protein was purified by immunoprecipitation.The lytic cycle of bacteriophage lambda concludes with the synchronous lysis of the host Escherichia coli cells and the efficient release of approximately 100 newly synthesized phage particles per cell. Two phage genes are sufficient for this synchronous lysis, as demonstrated by the cloning of the lambda lysis region into a plasmid under the transcriptional control of the lac operon (14,15). The lambda R gene encodes a soluble 17.5-kilodalton (kDa) transglycosylase that is synthesized in an active form in the cytoplasm and is capable of degrading the periplasmic peptidoglycan layer (4-7). The lambda S gene product increases the permeability of the inner membrane at 40 to 50 min postinfection and thereby initiates cell lysis (15,35,36,50). A third phage gene product, the Rz protein, is required for efficient lysis when the cells are grown in the presence of high concentrations of magnesium and is thought to encode an endopeptidase activity which cleaves peptide bonds that are present in the peptidoglycan layer (53). The simplest model for the mechanism of S gene-induced lysis is the formation or activation of a large nonspecific pore(s) through the inner membrane that allows the cytoplasmic R (and possibly Rz) gene product access to the periplasm at the time of lysis (50). This hypothesis accounts for the increased membrane permeability that was observed in the presence of the S protein and incorporates a reversible element (the opening and closing of the pore), which is compatible with experimental results that have been obtained by using the temperature-sensitive S allele (50).Initial reports identified both a 15-kDa acidic protein in the inner membrane fraction of lambda-infected cells and a 5.5-kDa protein found in phage particles as the products of the S gene (50,51 which suggests that there are two translational start sites controlled by an upstream stem-loop structure, resulting in the production of two proteins, S107 and S105, which differ by two N-terminal residues (8a). It has been proposed that the regulation of the timing of lysis might inv...
