The vanG glycopeptide resistance operon from Enterococcus faecalis revisited

Acquired VanG-type resistance to vancomycin (MIC = 16 mug ml(-1)) but susceptibility to teicoplanin in Enterococcus faecalis BM4518 and WCH9 is due to the inducible synthesis of peptidoglycan precursors ending in D-alanine-D-serine. The vanG cluster, assigned to a chromosomal location, was composed of genes recruited from various van operons. The 3' end encoded VanG, a D-Ala:D-Ser ligase, VanXY(G), a putative bifunctional D,D-peptidase and VanT(G), a serine racemase: VanG and VanT(G) were implicated in the synthesis of D-Ala:D-Ser as in VanC- and VanE-type strains. Upstream from the structural genes for these proteins were vanW(G) with unknown function and vanY(G) containing a frameshift mutation which resulted in premature termination of the encoded protein and accounted for the lack of UDP-MurNAc-tetrapeptide in the cytoplasm. Without the frameshift mutation, VanY(G) had homology with Zn2+ dependent D,D-carboxypeptidases. The 5' end of the gene cluster contained three genes vanU(G), vanR(G) and vanS(G) encoding a putative regulatory system, which were co-transcribed constitutively from the PYG promoter, whereas transcription of vanY(G),W-G,G,XYG,T-G was inducible and initiated from the P-YG promoter. Transfer of VanG-type glycopeptide resistance to E. faecalis JH2-2 was associated with the movement, from chromosome to chromosome, of genetic elements of c. 240 kb carrying also ermB-encoded erythromycin resistance. Sequence determination of the flanking regions of the vanG cluster in donor and transconjugants revealed the same 4 bp direct repeats and 22 bp imperfect inverted repeats that delineated the large element.