Dominance Relationships between Two Allelic Genes Controlling Glycosyltransferases with Different Substrate Specificity in Melandrium.

Genetical analysis showed that the genes g(G) and g(X), which control, respectively, the glucosylation and xylosylation of the 7-hydroxyl group of isovitexin in the petals of Melandrium, are alleles. In petal extracts of plants possessing the gene g(X) an enzyme was present which catalyzed the transfer of the xylose moiety of UDP-xylose to the 7-hydroxyl group of isovitexin. The xylosyl-transferase controlled by the gene g(X) had a "true K(m) value" of 0.77 mM for UDP-xylose. The "true K(m) value" for isovitexin was << 0.04 mM. The transfer of glucose from UDP-glucose to the 7-hydroxyl group of isovitexin is catalyzed by the enzyme controlled by gene g(G). In plants possessing both the alleles g(G) and g(X), only the gene product of g(G), i.e., isovitexin 7-O-glucoside, was found. In this respect g(G) is dominant over allele g(X). In petal extracts of these g(G)g(X) plants, however, besides UDP-glucose: isovitexin 7-O-glucosyltransferase, also UDP-xylose: isovitexin 7-O-xylosyltransferase could be detected. This means that the dominance is not a consequence of transcriptional and/or translational control. Enzyme kinetic experiments showed that inhibition of the xylosyltransferase by the endproduct of the glucosyltransferase did not occur. Comparison of the enzyme kinetic parameters revealed that the dominance is probably caused by differences in V(max) between the two enzymes, both working at saturating isovitexin concentrations. A competition model is suggested which explains why the amount of isovitexin 7-O-glucoside in g(G)g(G) plants and the amount of isovitexin 7-O-xyloside in g(X)g(X) plants are about the same, whereas in g(G)g(X) plants isovitexin 7-O-xyloside escapes detection. The differences in distribution of the isovitexin glycosylation genes in the two species M. album and M. dioicum are discussed.