Molecular structure of three mutations at the maize sugary1 locus and their allele-specific phenotypic effects

Starch production in all plants examined is altered by mutations of isoamyl ase-type starch-debranching enzymes (DBE), although how these proteins affe ct glucan, polymer assembly is not understood. Various allelic mutations in the maize (Zen mays) gene sugary1 (su1), which codes for an isoamylase-typ e DBE, condition distinct kernel phenotypes. This study characterized the r ecessive mutations su1-Ref, su1-R4582::Mu1, and su1-st, regarding their mol ecular basis, chemical phenotypes, and effects on starch metabolizing enzym es. The su1-R4582::Mu1 mutation is a null allele that abolishes transcript accumulation. The Su1-st mutation results from insertion of a novel transpo son-like sequence, designated Toad, ahich causes alternative without affect ing the Su1 mRNA level. The su1-R4582::Mu1 mutation is a null allele that a bolishes transcript accumulation. The su1-st mutation results from insertio n of a novel transposon-like sequence, designated Toad which causes alterna tive pre-mRNA splicing. Three su1-st mutant transcripts are produced, one t hat is nonfunctional and two that code for modified SU1 polypetides. The su 2-st mutation is dominant to the null allele su1-R4582::Mu1 but recessive t o su1-Ref, suggestive of complex effects involving quaternary structure of the SU1 enzyme. All three su1- alleles severely reduce or eliminate isoamyl ase-type DBE activity, although su1-st kernels accumulate less phytoglycoge n and Suc than su1-Ref or su1-R4587::Mu1 mutants. The chain length distribu tion of residual amylopectin is significantly altered by su1-Ref and su1-R4 582::Mu1 whereas su1-st has modest effects. These results, together with si ll allele-specific effects on other starch-metabolizing enzymes detected in zymograms, suggest that total DBE catalytic activity is the not the sole d eterminant of Su1 function and that specific interactions between SU1 and o ther components of the starch biosynthetic system are required.