Selectivity for d -Lactate Incorporation into the Peptidoglycan Precursors of Lactobacillus plantarum : Role of Aad, a VanX-Like d -Alanyl- d -Alanine Dipeptidase

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Abstract
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.