The size of the Wx protein produced by the wx-m-9 gene (McClintock, Carnegie Inst. Ann. Rep. #62) depends on the dosage of this controlling element mutant, in which Ac is inserted in the gene locus. In 20-day wx-m-9/wx/wx endosperm only a single Wx protein is produced as revealed by silver stain MDS acrylamide electrophoresis. The amount of protein produced is variable but it is of uniform size, slightly larger than the protein specified by the Wx allele. This protein is also produced by germinal revertants of wx-m-9 resulting from prezygotic transposition of Ac. In both wx-m-9/wx-m-9/wx and wx-m-9/wx-m-9/wx-m-9 20-day endosperm, a protein of this size is not detected and instead, one finds a double Wx protein band--one protein slightly larger and the other slightly smaller than the one-dose protein. The two protein bands are of about the same low intensity.
The wx-m-9 allele is not fully suppressed by the presence of the Ac element and some amylose is synthesized. As transpositions occur very late in two-dose wx-m-9 endosperm and practically not at all in three-dose wx-m-9 endosperm, it is proposed that the two Wx proteins in the double band are the products of the Ac "suppressed" gene and that the single intermediate sized Wx protein of wx-m-9/wx/wx endosperm is the product of the reverted gene in which Ac was transposed away from the locus. Analysis of our data further suggests that we are dealing with included genes, the Ac "gene" being inserted within the limits of the wx structural gene. As shown by McClintock (Carnegie Inst. Ann. Rep. #64) to be the case for wx-m-7, we propose that functioning of the Ac gene and the Wx gene are mutually exclusive. In one-dose wx-m-9, Ac is active early and the Wx gene is inactive early except in those cells in which TE had been transposed away from the locus. In two- and three-dose endosperm where Ac is not active early in endosperm development, the Wx gene functions early and the other two double band Wx proteins are produced from mRNA in which the Ac segment was excised. Similar results are obtained when the dosage of wx-m-9 is kept constant and Ac dosage varied by the use of other non-linked Ac elements.
Drew Schwartz and Craig S. Echt
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