The effect of Ac dosage on the production of multiple forms of Wx protein by the wx m-9 controlling element mutation in maize

Abstract

Summary. The WX m-9 autonomous controlling element mutation produces either a single or doublet type protein in 20 day endosperm, depending on the dosage of Ac. The single protein observed in one dose Ac endosperm is the product of wx m-9 to Wx revertants. The doublet observed in two and three dose Ac endosperm is the product of the Ac-containing, partially suppressed wx "-9 gene. Mutual exclusion of the activities of the wx m-9 gene and its included Ac element is postulated. A competition model is presented to account for the unexpected Ac dosage effect. One approach to the analysis of the Ac-Ds transposable controlling element system in maize is the comparison of the protein product of a gene suppressed by the controlling element with that of the unsuppressed gene (Hannah and Nelson 1976; Dooner and Nelson 1977). The Wx locus is especially well suited for such studies. This gene specifies a starch-bound glycosyl transferase which is responsible for the production of amylose in the endosperm and pollen (Nelson and Rines 1962; Tsai 1974). Gene activity can be analyzed at the enzymatic, phenotypic, and protein levels. The wx mutants m-l, m-6, and B4, mapped by Nelson (1968, 76), where a controlling element is inserted in the Wx gene, cannot be analyzed by this approach since they represent cases of complete gene suppression and no detectable gene product is produced (Echt and Schwartz 1981). The autonomous wx m-9 mutation, hereafter designated m-9, has an Ac controlling element at the Wx locus (McClintock 1963). This mutant only partially suppresses the activity of the Wx gene since the endosperm stains pale blue with iodine, indicative of the presence of a low amount of amylose. The m-9 mutant undergoes frequent reversions to Wx as well as mutations to a stable recessive wx allele which does not make amylose. This paper deals with an analysis of the proteins produced by the suppressed and revertant m-9 mutant. Evidence will be presented which suggests that Ac is located within the limits of the coding region of the Wx gene where it behaves as an intron in that its sequence is not represented in the protein specified by the m-9 gene.