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 essentia l for resistance to the glycopeptide antibiotic vancomycin, While this enzy matic activity has been typically associated with vancomycin- and teicoplai nin-resistant enterococci, we now report the identification of a D-Ala-D-Al a dipeptidase in the gramnegative species Salmonella enterica. The Salmonel la enzyme is only 36% identical to VanX but exhibits a similar substrate sp ecificity: it hydrolyzes D-Ala-D-Ala, or-Ala-or-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 tr ansfer because pcgL-hybridizing sequences were not detected in related bact erial species and the G+C content of the pcgL-containing region (41%) is mu ch lower than the overall G+C content of the Salmonella chromosome (52%). I n 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 dipeptida se 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, sug gesting that this dipeptidase participates in peptidoglycan metabolism.