A glucosyltransferase cDNA, RF5, was cloned from Oryza sativa using an RT-PCR strategy and expressed in Escherichia coli. Several flavonoids were tested for their ability to serve as substrates for RF5. RF5 effectively glucosylated kaempferol and quercetin to produce their 3-O-glucosides. Thus, RF5 could be defined as a flavonol 3-O-glucosyltransferase. E. coli cells expressing RF5 effectively converted 100 mu M of kaempferol and quercetin into their corresponding glucosides.