Abstract
Peptidoglycan (PG) is a protective sac-like exoskeleton present in most bacterial cell walls. It is a large, covalently crosslinked mesh-like polymer made up of many glycan strands cross-bridged to each other by short peptide chains. Because PG forms a continuous mesh around the bacterial cytoplasmic membrane, opening the mesh is critical to generate space for the incorporation of new material during its expansion. In Escherichia coli, the 'space-making activity' is known to be achieved by cleavage of crosslinks between the glycan strands by a set of redundant PG endopeptidases whose absence leads to rapid lysis and cell death. Here, we demonstrate a hitherto unknown role of glycan strand cleavage in cell wall expansion in E. coli. We find that overexpression of a membrane-bound lytic transglycosylase, MltD that cuts the glycan polymers of the PG sacculus rescues the cell lysis caused by the absence of essential crosslink-specific endopeptidases, MepS, MepM and MepH. We find that cellular MltD levels are stringently controlled by two independent regulatory pathways; at the step of post-translational stability by a periplasmic adaptor-protease complex, NlpI-Prc, and post-transcriptionally by RpoS, a stationary-phase specific sigma factor. Further detailed genetic and biochemical analysis implicated a role for MltD in cleaving the nascent uncrosslinked glycan strands generated during the expansion of PG. Overall, our results show that the combined activity of PG endopeptidases and lytic transglycosylases is necessary for successful expansion of the cell wall during growth of a bacterium.