Cloning and regiospecificity studies of two flavonoid glucosyltransferases from Allium cepa

Two UDP-glucose-dependent flavonoid glucosyltransferases (EC 2.4.1.-) isolated from the epidermal layer of yellow onion (Allium cepa) were functionally expressed in Escherichia coli and their substrate specificity investigated. The two enzymes exhibited different substrate- and regio-specificity profiles. A. cepa UGT73G1 used a wide range of different flavonoid substrates including flavonoids not naturally occurring in onion. Regiospecificity was indicated for hydroxyl-groups of the C-3, C-7 and C-4' positions of the flavan backbone structure to yield flavonoid mono- and diglucosides. In contrast, A. cepa UGT73J1 showed activity only with the flavonoid mono-glucoside isoquercitrin and the isoflavone aglycone genistein, with regiospecificity for the C-7 position. The regiospecificity for both enzymes included positions that are glucosylated in flavonoids of onion bulbs, indicating their involvement in flavonoid biosynthesis in A. cepa.