The identification and functional characterization of three liverwort class I O-methyltransferases

Previously it has been shown that the caffeoyl coenzyme A O-methyltransferase (CCoAOMT) type enzyme PaF6OMT, synthesized by the liverwort Plagiochasma appendiculatum Lehm. & Lindenb., (Aytoniaceae), interacts preferentially with 6-OH flavones. To clarify the biochemistry and evolution of liverwort OMTs, a comparison was made between the nucleotide sequence and biological activity of PaF6OMT and those of three of its homologs MpOMT1 (from Marchantia paleacea Bertol., (Marchantiaceae)), MeOMT1 (Marchantia emarginata Reinw et al., (Marchantiaceae)) and HmOMT1 (Haplomitrium mnioides (Lindb.) Schust., (Haplomitriaceae)). The four genes shared >60% level of sequence identity with one another but a <20% level of similarity with typical CCoAOMT or CCoAOMT-like sequences; they clustered with genes encoding animal catechol methyltransferases. The recombinant OMTs recognized phenylpropanoids, flavonoids and coumarins as substrates, but not catechol. MpOMT1 and PaF6OMT exhibited some differences with respect to their substrate preference, and the key residues underlying this preference were identified using site-directed mutagenesis. The co-expression of MpOMT1 and the Arabidopsis thaliana gene encoding S-adenosyl-L-methionine synthase in Escherichia coli was shown to be an effective means of enhancing the production of the pharmacologically active compounds scopoletin and oroxylin A. Liverwort OMTs are thought likely to represent an ancestral out-group of bona fide higher plant CCoAOMTs in evolution and have the potential to be exploited for the production of methylated flavones and coumarins.