Nat. Microbiol. doi:10.1038/s41564-017-0023-4 (2017)

The bacterial cell wall consists of peptidoglycan (PG) and is the major obstacle in the release of newly synthesized bacteriophages. In single-strand RNA (ssRNA) phages, where replication is extremely mutagenic, diverse lysis genes have evolved at different sites within phage genomes, and their characterization could lead to new antibacterial strategies. Chamakura et al. observed that expression of the lysis gene lysM from ssRNA phage M causes morphological defects that are consistent with inhibition of PG biosynthesis. To identify the Escherichia coli target, the authors co-expressed lysM with plasmids containing random E. coli genome fragments and isolated lysis-resistant clones containing the murJ gene. murJ encodes a flippase that translocates the final PG precursor lipid II across the cytoplasmic membrane. The identification of LysM-resistant MurJ mutants and in vivo flippase assays provided further evidence that LysM specifically blocks lipid II flipping. Moreover, in vivo thiol-modification experiments in the presence and absence of LysM indicated that LysM binding could inactivate MurJ by trapping it in one of its two flipping cycle conformations. Thus, LysM adds to the growing repertoire of viral proteins that block PG biosynthesis steps and may facilitate the development of novel antibiotics.