Role of endopeptidases in peptidoglycan synthesis mediated by alternative cross‐linking enzymes in Escherichia coli

Abstract

Bacteria resist to the turgor pressure of the cytoplasm through a net-like macromolecule, the 13 peptidoglycan, made of glycan strands connected via peptides cross-linked by penicillin-binding 14 proteins (PBPs). We recently reported the emergence of β-lactam resistance resulting from a 15 bypass of PBPs by the YcbB L,D-transpeptidase (LdtD), which form chemically distinct 3→3 cross- 16 17 links compared to 4→3 formed by PBPs. Here we show that peptidoglycan expansion requires controlled hydrolysis of cross-links and identify amongst eight endopeptidase paralogues the 18 minimum enzyme complements essential for bacterial growth with 4→3 (MepM) and 3→3 19 (MepM and MepK) cross-links. Purified Mep endopeptidases unexpectedly displayed a 4→3 and 20 3→3 dual specificity implying recognition of a common motif in the two cross-link types. 21 Uncoupling of the polymerization of glycan chains from the 4→3 cross-liking reaction was found 22 to facilitate the bypass of PBPs by YcbB. These results illustrate the plasticity of the peptidoglycan 23 polymerization machinery in response to the selective pressure of β-lactams