Dominance relations between isovitexin: 7-0-glycosyltransferase alleles in Melandrium

Summary

In a hybrid swarm of Melandrium dioicum and Melandrium album, a plant was found in which the genes g ^G and g ^X, controlling an isovitexin 7-O-glucosyltransferase, respectively an isovitexin 7-O-xylosyltransferase, were co-dominant. In earlier experiments, gene g ^G was always dominant over its allele g ^X. Genetical and biochemical analysis revealed that this co-dominance was caused by an allele of g ^X, further designated g ^X′. Comparison of the enzyme-kinetic properties of the g ^X′ controlled xylosyltransferase with the g ^X controlled xylosyltransferase revealed no difference in substrate affinity for UDP-xylose nor for isovitexin. However, the maximal activity of the g ^X′ controlled transferase was 8+9 times higher than the g ^X controlled activity. These results support the competition model previously proposed to explain the dominance relationship between g ^G and g ^X. This model was based on the assumptions that 1) both the xylosyl- and the glucosyltransferase work at saturating isovitexin concentration and that 2) the lower V _max of the xylosyltransferase, makes this enzyme a poorer competitor for isovitexin than the glucosyltransferase.