CHARACTERIZATION OF DULL1, A MAIZE GENE CODING FOR A NOVEL STARCH SYNTHASE

The maize dull1 (du1) gene is a determinant of the structure of endosp erm starch, and du1- mutations affect the activity of two enzymes invo lved in starch biosynthesis, starch synthase II (SSII) and starch bran ching enzyme IIa (SBEIIa). Six novel du1- mutations generated in Mutat or-active plants were identified. A portion of the du1 locus was clone d by transposon tagging, and a nearly full-length Du1 cDNA sequence wa s determined. Du1 codes for a predicted 1674-residue protein, comprisi ng one portion that is similar to SSIII of potato, as well as a large unique region. Du1 transcripts are present in the endosperm during the time of starch biosynthesis, but the mRNA was undetectable in leaf or root tissue. The predicted size of the Du1 gene product and its expre ssion pattern are consistent with those of maize SSII. The Du1 gene pr oduct contains two repeated regions in its unique N terminus. One of t hese contains a sequence identical to a conserved segment of SBEs. We conclude that Du1 codes for a starch synthase, most likely SSII, and t hat secondary effects of du1- mutations, such as reduction of SBEIIa, result from the primary deficiency in this starch synthase.