The lytic transglycosylase MltA participates in turnover of septal peptidoglycan in Escherichia coli

ABSTRACT Daughter cell separation in Escherichia coli is driven primarily by two classes of peptidoglycan (PG) hydrolases that work in tandem: N-acetylmuramoyl–L-alanine amidases that strip stem peptides from the PG glycan backbone and lytic transglycosylases (LTs) that break down the PG glycan backbone. Although the relevant amidases have been known for years, which of E. coli’s eight LTs contribute to this process is less clear. Because the amidases process PG first, the relevant LTs must utilize peptide-free or “denuded” glycan substrates (dnGs). MltA is one of the few E. coli LTs that can break down peptide-free PG glycans in vitro, but its precise physiological roles are not known. Here we show MltA localizes to the division site in constricting E. coli cells and cells lacking MltA accumulated dnGs in septal PG. We found that MltA binds to the anhydroMurNAc ends of glycan chains, which raises the possibility that these structures are enriched in septal PG. Nevertheless, as reported previously, deletion of mltA does not impair daughter cell separation sufficiently to cause a chaining phenotype. Overall, our findings demonstrate that MltA is a physiologically relevant peptidoglycan hydrolase for cell division in E. coli. IMPORTANCE How bacteria coordinate synthesis and cleavage of septal peptidoglycan remains poorly understood, in part because some of the relevant enzymes have yet to be identified. Here we show that the E. coli lytic transglycosylase MltA is involved in cleaving septal peptidoglycan. Besides elucidating a physiological role for MltA, our work brings the field a step closer to identifying all of the proteins involved in cell division in an important model organism.