A Periplasmic d-Alanyl-d-Alanine Dipeptidase in the Gram-Negative Bacterium Salmonella enterica

The VanX protein is a d-alanyl-d-alanine (d-Ala-d-Ala) dipeptidase essential for resistance to the glycopeptide antibiotic vancomycin. While this enzymatic activity has been typically associated with vancomycin- and teicoplainin-resistant enterococci, we now report the identification of a d-Ala-d-Ala dipeptidase in the gram-negative species Salmonella enterica. The Salmonella enzyme is only 36% identical to VanX but exhibits a similar substrate specificity: it hydrolyzes d-Ala-d-Ala, dl-Ala-dl-Phe, and d-Ala-Gly but not the tripeptides d-Ala-d-Ala-d-Ala and dl-Ala-dl-Lys-Gly or the dipeptides l-Ala-l-Ala, N-acetyl-d-Ala-d-Ala, and l-Leu-Pro. The Salmonella dipeptidase gene, designated pcgL, appears to have been acquired by horizontal gene transfer because pcgL-hybridizing sequences were not detected in related bacterial species and the G+C content of the pcgL-containing region (41%) is much lower than the overall G+C content of the Salmonella chromosome (52%). In contrast to wild-type Salmonella, a pcgL mutant was unable to use d-Ala-d-Ala as a sole carbon source. The pcgL gene conferred d-Ala-d-Ala dipeptidase activity upon Escherichia coli K-12 but did not allow growth on d-Ala-d-Ala. The PcgL protein localizes to the periplasmic space of Salmonella, suggesting that this dipeptidase participates in peptidoglycan metabolism.