Serine/Threonine Protein Phosphatase-Mediated Control of the Peptidoglycan Cross-Linking
l
,
d
-
Transpeptidase Pathway in Enterococcus faecium
Authors: Sacco Emmanuelle , Cortes Mélanie , Josseaume Nathalie , Rice Louis B. , Mainardi Jean-Luc , Arthur Michel 
Published: July 2014

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.

IMPORTANCE
The mechanism of acquisition of high-level ampicillin resistance involving bypass of the penicillin-binding proteins (PBPs) by
l
,
d
-
transpeptidase Ldt
fm
was incompletely understood, as production of tetrapeptide precursors following transcriptional activation of the
ddc
locus by the DdcRS two-component regulatory system was necessary but not sufficient for full activation of the
l
,
d
-
transpeptidation pathway. Here, we identified the release of a negative control of Ser/Thr protein phosphorylation mediated by phosphatase StpA as the additional factor essential for ampicillin resistance. Thus, bypass of PBPs by Ldt
fm
requires the modification of signal transduction regulatory systems without any gain of function by mutational alteration of peptidoglycan biosynthetic enzymes. In contrast, previously characterized mechanisms of antibiotic resistance involve horizontal gene transfer and mutational alteration of drug targets. Activation of the
l
,
d
-
transpeptidation pathway reported in this study is an unprecedented mechanism of emergence of a new metabolic pathway since it involved the recruitment of preexisting functions following modifications of regulatory circuits.