A single amino acid determines position specificity of an Arabidopsis thaliana CCoAOMT-like O-methyltransferase.

Caffeoyl-coenzyme A O-methyltransferase (CCoAOMT)-like proteins from plants display a conserved position specificity towards the meta-position of aromatic vicinal dihydroxy groups, consistent with the methylation pattern observed in vivo. A CCoAOMT-like enzyme identified from Arabidopsis thaliana encoded by the gene At4g26220 shows a strong preference for methylating the para position of flavanones and dihydroflavonols, whereas flavones and flavonols are methylated in the meta-position. Sequence alignments and homology modelling identified several unique amino acids compared to motifs of other CCoAOMT-like enzymes. Mutation of a single glycine, G46 towards a tyrosine was sufficient for a reversal of the unusual para- back to meta-O-methylation of flavanones and dihydroflavonols.