Characterization of the Maize Gene sugaryl , a of Starch Composition in Kernels Determinant

In maize kernels, mutations in the gene sugaryl (sul) result in (1) increased sucrose concentration; (2) decreased concentration of amylopectin, the branched component of starch; and (3) accumulation of the highly branched glucopolysaccharide phytoglycogen. To investigate further the mechanisms of storage carbohydrate synthesis in maize, part of the sul gene locus and a cDNA copy of the sul transcript were characterized. Five new sul mutations were isolated in a Mutator background, and the mutant allele sul-R4582::Mul was isolated by transposon tagging. The identity of the cloned element as the sul gene locus was confirmed by the cosegregation of restriction fragment length polymorphisms in the same or nearby genomic intervals with three additional, independent sul mutations. Pedigree analysis was also used to confirm the identity of sul. A 2.8-kb mRNA that is homologous to the cloned gene was detected in maize kernels, and a 2.7-kb cDNA clone was isolated based on hybridization to the genomic DNA. Specific portions of the cDNA hybridized with multiple segments of the maize genome, suggesting that sul is part of a multigene family. The cDNA sequence specified a polypeptide of at least 742 amino acids, which is highly similar in amino acid sequence to bacterial enzymes that hydrolyze a-(l-%) glucosyl linkages of starch. Therefore, debranching of glucopolysaccharides is seemingly part of the normal process of starch biosynthesis, and the final degree of branch linkages in starch most likely arises from the combined actions of branching and debranching enzymes.