Identification and molecular characterization of shrunken-2 cDNA clones of maize

Mutation at the shrunken-2 (Sh2) locus of maize, a gene described more than 40 years ago, greatly reduces starch levels in the endosperm through its effect on the starch synthetic enzyme ADP-glucose pyrophosphorylase, an enzyme thought to be regulatory in this biosynthetic pathway. Although our previous work has suggested that Sh2 is a structural gene for this enzyme, we have also reported data compatible with Sh2 acting post-transcriptionally. In this study, we took advantage of a transposable element-induced Sh2 allele, its progenitor, and revertants to identify a clone for this locus. Although the cloning and identification were done independently of any knowledge concerning the product of this gene, examination of the deduced amino acid sequence revealed much similarity to known ADP-glucose pyrophosphorylase subunits of plants and bacteria, including regions involved in substrate binding and activator binding. Little sequence similarity, however, was found at the DNA level. These observations provide direct evidence that Sh2 encodes a subunit for endosperm ADP-glucose pyrophosphorylase. Analysis of several phenotypically wild-type alleles arising from a mutable sh2-Ds allele revealed one unexpected case in which DNA sequences of Sh2 were rearranged in comparison with the progenitor Sh2. In contrast to wild type, the Ds-induced sh2 allele conditions at least two transcripts in the endosperm.