The lysis protein E of φX174 is a specific inhibitor of the MraY-catalyzed step in peptidoglycan synthesis

Coliphage phi X174 encodes a single lysis protein, E, a 91-amino acid membrane protein. Dominant mutations have been isolated in the host gene mraY that confer E resistance. mraY encodes translocase I, which catalyzes the formation of the first lipid intermediate in bacterial cell wall synthesis, suggesting a model in which E inhibits MraY and promotes cell lysis in a manner analogous to cell wall synthesis inhibitors like penicillin. To test this model biochemically, we monitored the effect of E on cell wall synthesis in vivo and in vitro. We find that expression of Emyc, encoding an epitope-tagged E protein, from a multicopy plasmid inhibits the incorporation of [H-3]diaminopimelic acid into cell wall and leads to a profile of labeled precursors consistent with MraY inhibition. Moreover, we find that membranes isolated after Emyc expression are drastically reduced in MraY activity, whereas the activity of Rfe, an enzyme in the same superfamily, was unaffected. We therefore conclude that E is indeed a cell wall synthesis inhibitor and that this inhibition results from a specific block at the MraY-catalyzed step in the pathway.