Characterization of two cDNA clones which encode O-methyltransferases for the methylation of both flavonoid and phenylpropanoid compounds.

Enzymatic O-methylation of phenylpropanoid and flavonoid compounds is believed to be catalyzed by distinct classes of O-methyltransferases [EC 2.1.1.6x]. The O-methylated derivatives of phenylpropanoids and flavonoids play an important role in lignification and as antimicrobial compounds, respectively. Two cDNA clones, OMT1 and OMT2, which differ in three amino acid residues were isolated and characterized from the semiaquatic freshwater weed Chrysosplenium americanum (Saxifragaceae). These two novel cDNA clones encode enzymes which catalyze the 3'-O-methylation of the flavonoid aglycones luteolin and quercetin, although they also catalyze the efficient 3/5-O-methylation of the phenylpropanoids caffeic and 5-hydroxyferulic acids, respectively. Both recombinant proteins were partially purified from an Escherichia coli expression system and their kinetic parameters were compared using two flavonoids and two phenylpropanoids as substrates. Although both gene products methylate caffeic acid and 5-hydroxyferulic acid to a similar extent, they exhibit a threefold higher affinity for and a four- to sixfold increase in turnover of flavonoid compounds. The gene product of OMT1 accepts the flavonoid substrates luteolin and quercetin for methylation at a higher rate than that of OMT2, as indicated by a two- to threefold increase in its Vmax values and turnover ratios. The fact that C. americanum accumulates a variety of highly methylated flavonols and exhibits little lignification suggests that these two flavonoid OMT clones have retained their ability to O-methylate phenylpropanoids as well. These results are discussed in relation to differences in the amino acid sequences of these two clones, as well as with other O-methyltransferases, and the evolutionary divergence of these genes in plants.