Coordinated and distinct LD-transpeptidase-independent roles of peptidoglycan carboxypeptidases DacC and DacA in stress adaptation and cell shape maintenance

Peptidoglycan (PG) is an essential bacterial architecture pivotal for shape maintenance and adaptation to osmotic stress. Although PG synthesis and modification are tightly regulated under harsh environmental stresses, few related mechanisms have been investigated. In this study, we aimed to investigate the coordinated and distinct roles of the PG carboxypeptidases DacC and DacA, in adaptation to alkaline and salt stresses and shape maintenance in Escherichia coli. We found that DacC is an alkaline PG carboxypeptidase, whose enzyme activity and protein stability are significantly enhanced under alkaline stress. Both DacC and DacA were required for bacterial growth under alkaline stress, whereas only DacA was required for the adaptation to salt stress. Under normal growth conditions, only DacA was necessary for cell shape maintenance, while under alkaline stress conditions, both DacA and DacC were necessary for cell shape maintenance, but their roles were distinct. Notably, all these roles of DacC and DacA were independent of LD-transpeptidases, which are necessary for the formation of PG 3-3 crosslinks and covalent bonds between PG and the outer membrane lipoprotein Lpp. Instead, DacC and DacA interacted with penicillin-binding proteins (PBPs), DD-transpeptidases, mostly in a C-terminal domain-dependent manner, and these interactions were necessary for most of their roles. Collectively, our results demonstrate the coordinated and distinct novel roles of PG carboxypeptidases in stress adaptation and shape maintenance and provide novel insights into the cellular functions of PG carboxypeptidases associated with PBPs.