Selectivity for D-lactate incorporation into the peptidoglycan precursors of Lactobacillus plantarum: role of Aad, a VanX-like D-alanyl-D-alanine dipeptidase.

Lactobacillus plantarum produces peptidoglycan precursors ending in D-lactate instead of D-alanine, making the bacterium intrinsically resistant to vancomycin. The ligase Ddl of L. plantarum plays a central role in this specificity by synthesizing D-alanyl-D-lactate depsipeptides that are added to the precursor peptide chain by the enzyme MurF. Here we show that L. plantarum also encodes a D-Ala-D-Ala dipeptidase, Aad, which eliminates D-alanyl-D-alanine dipeptides that are produced by the Ddl ligase, thereby preventing their incorporation into the precursors. Although D-alanine-ended precursors can be incorporated into the cell wall, inactivation of Aad failed to suppress growth defects of L. plantarum mutants deficient in d-lactate-ended precursor synthesis.