Plant cytochrome P450s interact with a flavoprotein, NADPH-cytochrome P450 reductase (CPR), to transfer electrons from NADPH. The gene for rice P450 reductase (RCPR) was cloned and expressed in Saccaromyces cerevisiae, where the specific activity of the expressed RPCR was 0.91 U/mg protein. When isoflavone synthase gene (IFS) from red clover, used as a model system of plant cytochrome P450, was co-expressed with RCPR in yeast, the production of genistein from naringein increased about 4.3-fold, indicating that the RCPR efficiently interacts with cytochrome P450 to transfer electrons from NADPH.