cDNA cloning and characterization of a 3'/5'-O-methyltransferase for partially methylated flavonols from Chrysosplenium americanum.

Enzymatic O-methylation of plant secondary metabolites is an important mechanism for the inactivation of reactive hydroxyl groups and for the modification of their solubility. A cDNA clone (pFOMT3') encoding the gene for the 3'/5'-O-methylation of partially methylated flavonols was isolated from Chrysosplenium americanum (Saxifragaceae). We used a PCR fragment obtained with degenerate oligonucleotides designed from conserved regions of various O-methyltransferases (OMTs). The pFOMT3' cDNA sequence shows about 67-85% similarity to other plant OMT sequences. The recombinant protein expresses strict specificity for positions 3'/5' (meta) of partially methylated flavonols, but does not accept quercetin or caffeic acid for further methylation. Southern blot analysis of the genomic DNA probed with an OMT sequence suggests the presence of a number of related genes in this species, consistent with the multiple enzymatic methylations involved in the biosynthesis of polymethylated flavonols in this plant.