Molecular mechanism of resistance to vancomycin in Enterococcus gallinarum

• Main Author: Arias, C. A.
• Published: University of Cambridge 2000
• Subjects: 572.8
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596145

The <I>vanC</I> cluster of <I>Enterococcus gallinarum </I>BM4174 contains five genes: <I>vanC-1, vanXY<SUB>C</SUB>, vanT, vanRc</I> and <I>vanS<SUB>C</SUB>. </I>Three genes are sufficient for vancomycin resistance: <I>vanC-1</I> encodes a ligase that synthesises the dipeptide D-Ala-D-Ser for addition to UDP-MurNAc-tripeptide; <I>vanXY<SUB>C</SUB> </I>encodes a D,D-dipeptidase/carboxypeptidase that hydrolyses D-Ala-D-Ala and removes D-Ala from UDP-MurNAc-pentapeptide[D-Ala], and <I>vanT</I> encodes a membrane-bound serine racemase that provides D-Ser for the synthetic pathway. The three genes are clustered: the start codons of <I>vanXY<SUB>C</SUB></I> and <I>vanT</I> overlap the termination codon of <I>vanC-1</I> and <I>vanXY<SUB>C</SUB> </I>respectively. The serine racemase VanT is a 698 amino acid polypeptide with an amino terminal domain containing ten predicted transmembrane segments. The protein contains a highly conserved pyridoxal phosphate attachment site in the C-terminal domain, typical of alanine racemases. Comparative modelling of the deduced C-terminal domain was based on the alignment of VanT with the Alr alanine racemase from <I>Bacillus stearothermophilus</I>. The model reveals that almost all critical amino acids in the active site of Alr are conserved in VanT, indicating that the C-terminal domain of VanT is likely to adopt a three-dimensional structure similar to that of Alr and that the protein could exist as a dimer. The purified soluble domain of VanT also exhibited both serine and alanine racemase activities. Two genes which encodes proteins with homology to the VanS-VanR two-component regulatory system are present downstream from the resistance genes. All residues with important functions in response regulators and histidine kinases are conserved in VanR<SUB>C</SUB> and VanS<SUB>C</SUB> respectively.