Serine/Threonine Protein Phosphatase-Mediated Control of the Peptidoglycan Cross-Linking l,d-Transpeptidase Pathway in Enterococcus faecium

The last step of peptidoglycan polymerization involves two families of unrelated transpeptidases that are the essential targets of β-lactam antibiotics. d,d-transpeptidases of the penicillin-binding protein (PBP) family are active-site serine enzymes that use pentapeptide precursors and are the main or exclusive cross-linking enzymes in nearly all bacteria. However, peptidoglycan cross-linking is performed mainly by active-site cysteine l,d-transpeptidases that use tetrapeptides in Mycobacterium tuberculosis, Clostridium difficile, and β-lactam-resistant mutants of Enterococcus faecium. We have investigated reprogramming of the E. faecium peptidoglycan assembly pathway by a switch from pentapeptide to tetrapeptide precursors and bypass of PBPs by l,d-transpeptidase Ldt(fm). Mutational alterations of two signal transduction systems were necessary and sufficient for activation of the l,d-transpeptidation pathway, which is essentially cryptic in wild-type strains. The first one is a classical two-component regulatory system, DdcRS, that controls the activity of Ldt(fm) at the substrate level. As previously described, loss of DdcS phosphatase activity leads to production of the d,d-carboxypeptidase DdcY and conversion of the pentapeptide into the tetrapeptide substrate of Ldt(fm). Here we show that full bypass of PBPs by Ldt(fm) also requires increased Ser/Thr protein phosphorylation resulting from impaired activity of phosphoprotein phosphatase StpA. This enzyme negatively controlled the level of protein phosphorylation both by direct dephosphorylation of target proteins and by dephosphorylation of its cognate kinase Stk. In combination with production of DdcY, increased protein phosphorylation by this eukaryotic-enzyme-like Ser/Thr protein kinase was sufficient for activation of the l,d-transpeptidation pathway in the absence of mutational alteration of peptidoglycan synthesis enzymes.