A novel and conserved cell wall enzyme that can substitute for the Lipid II synthase MurG

The cell wall is a stress-bearing structure and a unifying trait in bacteria. Without exception, synthesis of the cell wall involves formation of the precursor molecule Lipid II by the activity of the essential biosynthetic enzyme MurG, which is encoded in the division and cell wall synthesis (dcw) gene cluster. Here we present the discovery of a novel cell wall enzyme that can substitute for MurG. A mutant of Kitasatospora viridifaciens lacking a significant part of the dcw cluster including murG surprisingly produced Lipid II and wild-type peptidoglycan. Genomic analysis identified a distant murG paralogue, which encodes a putative enzyme that shares only around 31% aa sequence identity with MurG. We show that this enzyme can replace the canonical MurG, and we therefore designated it MurG2. Orthologues of murG2 are present in 38% of all genomes of Kitasatosporae and members of the sister genus Streptomyces. CRISPRi experiments showed that K. viridifaciens murG2 can also functionally replace murG in Streptomyces coelicolor, thus validating its bioactivity and demonstrating that it is active in multiple genera. Altogether, these results identify MurG2 as a bona fide Lipid II synthase, thus demonstrating plasticity in cell wall synthesis.