Some polyacylated anthocyanins, anthocyanins containing two or more aromatic acyl groups, are blue in color within a wide range of pH values, including acidic and neutral conditions even in the absence of any co-pigments or metal ions. We engineered a mutant of a malonyl-CoA:anthocyanin 5-O-glucoside-6″′-O-malonyltransferase of Salvia splendens (Ss5MaT1), Ss5AT306, which has an acquired specificity for hydroxycinnamoyl-CoA and is able to produce novel polyacylated anthocyanins. Ss5AT306 showed 6″′-O-hydroxycinnamoyltransferase activity toward some anthocyanins in addition to the 6″′-O-malonyltransferase activity retained (relative activities for acyl transfer to shisonin: malonyl-CoA, 100%; p-coumaroyl-CoA, 132%; caffeoyl-CoA, 103%). This alteration of acyl-donor specificity was achieved by the substitutions of only three contiguous amino acid residues, Val39-Arg40-Arg41, to the corresponding residues of anthocyanin aromatic acyltransferases, Met-Leu-Gln, suggesting that these amino acid residues are key residues governing the malonyl-CoA specificity of Ss5MaT1. Through the use of Ss5AT306, a novel polyacylated anthocyanin, p-coumaroylshisonin, with one aromatic acyl group in each of the 3-O-glucosyl and 5-O-glucosyl moieties of its structure, was produced and characterized in vitro. p-Coumaroylshisonin was bluer in color and displayed stronger color intensity than did shisonin, implying that accumulation of polyacylated anthocyanins such as p-coumaroylshisonin cause the modulation of flower colors.
2007 by Japanese Society for Plant Cell and Molecular Biology