CHARACTERIZATION OF THE MAIZE GENE SUGARY1, A DETERMINANT OF STARCH COMPOSITION IN KERNELS

In maize kernels, mutations in the gene sugary1 (su1) result in (1) in creased sucrose concentration; (2) decreased concentration of amylopec tin, the branched component of starch; and (3) accumulation of the hig hly branched gluco-polysaccharide phytoglycogen. To investigate furthe r the mechanisms of storage carbohydrate synthesis in maize, part of t he su1 gene locus and a cDNA copy of the su1 transcript were character ized. Five new su1 mutations were isolated in a Mutator background, an d the mutant allele su1-R4582::Mu1 was isolated by transposon tagging, The identity of the cloned element as the su1 gene locus was confirme d by the cosegregation of restriction fragment length polymorphisms in the same or nearby genomic intervals with three additional, independe nt su1 mutations. Pedigree analysis was also used to confirm the ident ity of su1.A 2.8-kb mRNA that is homologous to the cloned gene was det ected in maize kernels, and a 2.7-kb cDNA clone was isolated based on hybridization to the genomic DNA. Specific portions of the cDNA hybrid ized with multiple segments of the maize genome, suggesting that su1 i s part of a multigene family. The cDNA sequence specified a polypeptid e of at least 742 amino acids, which is highly similar in amino acid s equence to bacterial enzymes that hydrolyze alpha(1-->6) glucosyl link ages of starch. Therefore, debranching of glucopolysaccharides is seem ingly part of the normal process of starch biosynthesis, and the final degree of branch linkages in starch most likely arises from the combi ned actions of branching and debranching enzymes.