Genomic Nucleotide Sequence of a Wild-Type Shrunken-2 Allele of Zea mays.

The maize (Zea mays) endosperm enzyme, ADP-glucose pyrophosphorylase, which produces ADP-glucose from ATP and glucose-l-phosphate, is an important enzyme in the synthesis of starch. ADP-glucose arising from the action of this enzyme is the major, if not sole, donor of glucose for starch biosynthesis. The enzyme is composed oftwo dissimilar subunits encoded by the two unlinked genes, Shrunken-2 (Sh2) and Brittle-2 (Bt2) (1, 2). The enzyme is allosterically activated by 3-phosphoglyceric acid and inhibited by phosphate (3). Although it remains an open question whether these allosteric properties are physiologically relevant or whether they simply reflect the evolutionary history of the two structural genes (4), there does exist, nevertheless, much interest in determining whether genetic modification of this enzyme could lead to increased rates of starch biosynthesis in the maize endosperm. Because endosperm starch content comprises approximately 70% of the dry weight of the seed, alterations in starch biosynthesis would clearly affect corn yield. As a first step toward such experiments, we have isolated and sequenced genomic clones of Sh2. The structure of the gene (Table I, Fig. 1) is based on sequence analysis of three overlapping clones isolated from a Black Mexican Sweet genomic library. Exonic sequences were defined by comparison with the cDNA sequenced previously (2) and further sequencing ofcDNAs subsequently isolated. These sequences