Gene dosage at the amylose-extender locus of maize: effects on the levels of starch branching enzymes.

Soluble starch branching enzymes and starch synthases from maize kernels of differing dosage of the ae locus were purified by DEAE-cellulose chromatography. A near-linear relationship between increasing dosage of the dominate amylose-extender allele (Ae) and branching enzyme IIb activity was found. In contrast, levels and properties of branching enzymes I and IIa, as well as the citrate-stimulated and primer-requiring starch synthases, remained unchanged. The near-linear increase in branching enzyme IIb activity with increasing doses of the Ae allele is consistent with the hypothesis that ae is the structural gene coding for branching enzyme IIb.