The vanG glycopeptide resistance operon from Enterococcus faecalis revisited

Abstract

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 Zn 2+ dependent D,D-carboxypeptidases. The 5′ end of the gene cluster contained three genes vanUG, vanRG and vanS G 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.