抄録
We identified two glycosyltransferases that contribute to the structural diversification of flavonol glycosides in grapevine ( Vitis vinifera)-glycosyltransferase 5 (VvGT5) and VvGT6. Biochemical analyses showed that VvGT5 is a UDP-glucuronic acid: flavono1-3-0- glucuronosyltransferase (GAT), and VvGT6 is a bifunctional UDP-glucose/UDP-galactose:flavono1-3- O-glucosyltransferase/galactosyltransferase. The VvGT5 and VvGT6 genes have very high sequence similarity (91%) and are located in tandem on chromosome 11, suggesting that one of these genes arose from the other by gene duplication. Both of these enzymes were expressed in accordance with flavonol synthase gene expression and flavonoid distribution patterns in this plant, corroborating their significance in flavonol glycoside biosynthesis. The determinant of the specificity of VvGT5 for UDP-glucuronic acid was found to be Arg140. We also analyzed the determinants of the sugar donor specificity of VvGT6. G1n373 and Prol9 were found to play important roles in the bifunctional specificity of the enzyme. The results presented here suggest that the sugar donor specificities of these VvGTs could be determined by a limited number of amino acid substitutions in the primary structures of protein duplicates, illustrating the plasticity of plant glycosyltransferases in acquiring new sugar donor specificities.
