Bacterial lysis induced by the expression of the cloned lysis gene of the RNA bacteriophage MS2 in Escherichia coli was shown to be under the same regulatory control mechanisms as penicillin-induced lysis. It was controlled by the stringent response and showed the phenomenon of tolerance when E. coli was grown at pH 5. Changes in the fine structure of the murein were found to be the earliest physiological changes in the cell, taking place 10 min before the onset of cellular lysis and inhibition of murein synthesis. Both the average length of the glycan strands and, with a time lag, the degree of cross-linkage were altered, indicating that a lytic transglycosylase and a DD-endopeptidase had been triggered. After extensive separation of the membranes by isopycnic sucrose gradient centrifugation, the lysis protein was present predominantly in the cytoplasmic membrane and in a fraction of intermediate density and, to a lesser degree, in the outer membrane, irrespective of the conditions of growth. However, only under lysis-permissive conditions could a 17% increase in the number of adhesion sites between the inner and outer membranes be observed. Thus, a causal relationship between lysis and the formation of lysis protein-induced adhesion sites seems to exist.Bacteriophages have developed various mechanisms of quite different complexity to escape from their host at a well-controlled time point during their infection cycle. Some of the small phages such as 4iX174 and MS2 have reduced their contribution to this important step to a single lysis gene. These genes code for small-molecular-weight membrane proteins. Their expression has been proven to be sufficient to induce lysis of the host (6,8,14,21,41). However, it is not clear how these proteins function on a molecular level or if host functions are involved as well.The lysis gene (L) of phage MS2 has been cloned, and a temperature-inducible expression system has been constructed (21). Intensive studies of the physiological functions of the host cell after lysis gene induction failed to detect any changes, including the rate of murein synthesis preceding the onset of bacterial lysis (18). The primary effect of the lysis protein, therefore, remains in question.For further elucidation of the mechanism of action of the MS2 lysis protein, its interaction with the bacterial cell wall has been investigated by determining its specific distribution in the bacterial cell wall and its structural effects on both the membranes and the murein sacculus. MATERIALS AND METHODSBacterial strains and plasmids. Escherichia coli M5219 (36) harboring a defective prophage which codes for a temperature-sensitive repressor (cI587) was transformed with either plasmid pMS16 or plasmid pMS20. Both plasmids are pPLa831 derived and contain an MS2 insert downstream of the PL. promoter. The insert in pMS16 is the EcoRI-BamHI segment at positions 869 to 2057 coding for the MS2 coat and lysis genes; the insert in pMS20 is the same as that in pMS16 but carries a small deletion (positions 1764 to 1822)...

The mode of action of the phage MS2 lysis protein seems not to involve a direct interaction with the murein synthesis machinery as is the case for lysis induced by β‐lactam antibiotics. Mutants with defects in various penicillin‐binding proteins, which are involved in murein synthesis, were found to show normal lysis sensitivity towards the cloned MS2 lysis protein. In addition, both processes, longitudinal growth of the murein sacculus in the presence of furazlocillin, cephalexin and nalidixic acid as well as spherical growth in the presence of mecillinam were sensitive to the phage lysis protein. No change in the capacity of the binding proteins to bind 14C‐labelled penicillin G was observed in the presence of the MS2 lysis gene product.