Dominance relationships between allelic glycosyltransferase genes in Melandrium: An enzyme-kinetic approach

In the petals of Melandrium the glycosylation of the 7-hydroxylgroup of isovitexin is governed by a series of 4 multiple alleles: g(G), g, g(X), and [Formula: see text]. Gene g(G) is the structural gene for UDP-glucose: isovitexin 7-0-glucosyltransferase; the alleles g(X) and [Formula: see text] are structural genes for UDP-xylose: isovitexin 7-0-xylosyltransferase. Gene g is inactive and does not produce a functional glycosyltransferase. In the presence of both gene g(G) and its allele g(X) the product of gene g(X) (isovitexin 7-0-xyloside) is not detectable. In this respect gene g(G) is dominant over its allele g(X). In petal extracts of these g(G)/g(X) plants, xylosyltransferase, as well as glucosyltransferase, can be detected. The dominance is therefore not a consequence of transcriptional and/or translational control. Enzyme kinetic experiments demonstrated that inhibition of xylosyltransferase by the end product of glucosyltransferase did not occur. Comparison of the enzyme kinetic parameters revealed that dominance is probably caused by differences in Vmax between the two enzymes, both working at saturating isovitexin concentrations. A competition model is proposed which explains why the amounts of isovitexin 7-0-xyloside in g(X)g(X) and isovitexin 7-0-glucoside in g(G)g(G) plants are about the same, whereas in g(G)g(X) plants isovitexin 7-0-xyloside escapes dejection. This competition model is supported by the enzyme kinetic results found with the codominant allele [Formula: see text].