Molecular cloning, expression and characterization of a glycosyltransferase from rice

Secondary plant metabolites undergo several modification reactions, including glycosylation. Glycosylation, which is mediated by UDP-glycosyltransferase (UGT), plays a role in the storage of secondary metabolites and in defending plants against stress. In this study, we cloned one of the glycosyltransferases from rice, RUGT-5 resulting in 40-42% sequence homology with UGTs from other plants. RUGT-5 was functionally expressed as a glutathione S-transferase fusion protein in Escherichia coli and was then purified. Eight different flavonoids were used as tentative substrates. HPLC profiling of reaction products displayed at least two peaks. Glycosylation positions were located at the hydroxyl groups at C-3, C-7 or C-4' flavonoid positions. The most efficient substrate was kaempferol, followed by apigenin, genistein and luteolin, in that order. According to in vitro results and the composition of rice flavonoids the in vivo substrate of RUGT-5 was predicted to be kaempferol or apigenin. To our knowledge, this is the first time that the function of a rice UGT has been characterized.