The Large Subunit of the Embryo Isoform of ADP Glucose Pyrophosphorylase from Maize

ADP Glc pyrophosphorylase (EC 2.7.7.27) is the enzyme catalyzing the first committed step of starch anabolism (reviewed by Sivak and Preiss, 1994). The enzyme in higher plants is a tetramer of two dissimilar subunits (reviewed in Sivak and Preiss, 1994). In Zeu mays, the enzyme has been shown to be present as kinetically distinguishable isozymes in different tissues (Preiss et al., 1971; reviewed by Sivak and Preiss, 1994). For maize, two genetic loci conditioning the presence of enzyme activity solely in the endosperm have been reported (reviewed by Sivak and Preiss, 1994), and clones corresponding to these loci have been isolated and sequenced (Bae et al., 1990; Shaw and Hannah, 1992). The molecular basis for the observed isozymes is not completely known, since (a) genetic loci conditioning the presence of enzyme activity in other tissues are not known but are presumed to exist because bt2 and sh2 influence the enzyme activity exclusively in the endosperm, and (b) the cDNAs or the genomic clones corresponding to these presumptive loci have been reported only for part of the cDNA encoding a small subunit of one of the leaf isoforms (Prioul et al., 1994). This report concerns the determined nucleotide sequence of the cDNA and deduced primary structure corresponding to the large subunit of the embryo isoform of ADP Glc pyrophosphorylase (Table I). A companion report will do the same for the small subunit of the embryo isoform of ADP Glc pyrophosphorylase. In conjunction with experiments published elsewhere (Giroux and Hannah, 1994), we conclude that this insert corresponds to almost, if not all, of the mRNA encoding the large subunit of the embryo ADP Glc pyrophosphorylase for three reasons. First, this insert recognizes an abundant mRNA in only the embryo (Giroux and Hannah, 1994).