The vanG glycopeptide resistance operon from Enterococcus faecalis revisited

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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 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.

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