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 th e modification of their solubility. A cDNA clone (pFOMT3') encoding th e gene for the 3'/5'-O-methylation of partially methylated flavonols w as isolated from Chrysosplenium americanum (Saxifragaceae). We used a PCR fragment obtained with degenerate oligonucleotides designed from c onserved regions of various O-methyltransferases (OMTs). The pFOMT3' c DNA sequence shows about 67-85% similarity to other plant OMT sequence s. The recombinant protein expresses strict specificity for positions 3'/5' (meta) of partially methylated flavonols, but does not accept qu ercetin or caffeic acid for further methylation. Southern blot analysi s of the genomic DNA probed with an OMT sequence suggests the presence of a number of related genes in this species, consistent with the mul tiple enzymatic methylations involved in the biosynthesis of polymethy lated flavonols in this plant.