Synthesis of the l-Alanyl-l-alanine Cross-bridge of Enterococcus faecalis Peptidoglycan*

The enzymatic synthesis of the completel-alanyl1-l-alanine2side chain of the peptidoglycan precursors of Enterococcus faecalis was obtained in vitro using purified enzymes. The pathway involved alanyl-tRNA synthetase and two ligases, BppA1 and BppA2, that specifically transfer alanine from Ala-tRNA to the first and second positions of the side chain, respectively. The structure of the UDP-N-acetylmuramoyl-l-Ala-γ-d-Glu-l-Lys(N ε-l-Ala1-l-Ala2)-d-Ala-d-Ala product of BppA1 and BppA2 was confirmed by mass spectrometry (MS) and MS/MS analyses. The peptidoglycan structure of the wild-type E. faecalis strain JH2-2 was determined by tandem reverse-phase high-pressure liquid chromatography-MS revealing that most muropeptides contained two l-alanyl residues in the cross-bridges and in the free N-terminal ends. Deletion of the bppA2 gene was associated with production of muropeptides containing a single alanyl residue at these positions. The relative abundance of monomers, dimers, trimers, and tetramers in the peptidoglycan of the bppA2mutant indicated that precursors containing an incomplete side chain were efficiently used by the dd-transpeptidases in the cross-linking reaction. However, the bppA2 deletion impaired expression of intrinsic β-lactam resistance suggesting that the low affinity penicillin-binding protein 5 did not function optimally with precursors substituted by a single alanine.