Functional Expression of an Arabidopsis cDNA Clone Encoding a Flavonol 3′-O-Methyltransferase and Characterization of the Gene Product

We report that the cDNA clone (Accession No. U70424), previously isolated from Arabidopsis thaliana as encoding a caffeic acid/5-hydroxyferulic acid O-methyltransferase (OMT) (1), has now been overexpressed in Escherichia coli BL21 and its recombinant protein identified as a novel flavonol 3′-OMT. It is, therefore, renamed AtOMT1. This cDNA clone has previously been identified on the basis of its 88% amino acid sequence similarity and 80% identity to the aspen bispecific lignin OMT (2), the type member of the group involved in lignin biosynthesis. Our data indicate that this novel OMT uses the flavonol quercetin as the preferred substrate, but neither of the hydroxycinnamic acids, caffeic or 5-hydroxyferulic, to any significant extent. This indicates that the high sequence similarity/identity of AtOMT1 to that of the aspen lignin OMT (2) is not sufficient to assign the function of this gene product.