The vanG glycopeptide resistance operon from Enterococcus faecalis revisited



Acquired VanG-type resistance to vancomycin (MIC = 16 µg 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, VanXYG, a putative bifunctional d,d-peptidase and VanTG, a serine racemase: VanG and VanTG 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 vanWG with unknown function and vanYG 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, VanYG had homology with Zn2+ dependent d,d-carboxypeptidases. The 5′ end of the gene cluster contained three genes vanUG, vanRG and vanSG encoding a putative regulatory system, which were co-transcribed constitutively from the PYG promoter, whereas transcription of vanYG,WG,G,XYG,TG was inducible and initiated from the PYG 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.