A cDNA encoding starch branching enzyme I from maize endosperm.

ADP-Glc pyrophosphorylase (EC 2.7.7.27), starch synthases (EC 2.4.1.21), and SBEs (EC 2.4.1.18) are the key enzymes in the pathway of plant starch biosynthesis. Starch is a polymer of Glc that exists as two fractions, amylose and amylopectin, in maize (Zea mays L.) kernel amyloplasts. The essentially linear polymer amylose contains a-1,4-linked Glc, whereas the branched polymer amylopectin contains 5% a-1,6-linked Glc in addition to linear regions of a-1,4-linked Glc. Amylopectin synthesis requires the action of SBE, which catalyzes the formation of a-1,6linkages (Borovsky et al., 1976; Boyer, 1985). The branching process involves two steps with the hydrolysis of an interna1 1,4-bond and the formation of a 1,6-bond using the linear chain (six to seven Glc units). Thus, branching enzymes are thought to interact with starch synthases in formation of amylopectin (Boyer and Preiss, 1981). Three SBE isozymes differing in enzymatic, chromatographic, and immunological properties have been resolved in maize endosperm, SBE I, SBE IIa, and SBE IIb (Boyer and Preiss, 1978a, 1978b; Boyer and Fisher, 1984; Guan and Preiss, 1993; Takeda et al., 1993). Recently, analysis of SBE I, SBE IIa, and SBE IIb revealed that SBE I may preferentially branch long chains of a-glucan, whereas SBE IIa and SBE IIb may play a different role in branching short chains during starch biosynthesis (Takeda et al., 1993). We previously reported the cloning of a cDNA encoding SBE I1 from maize endosperm (Fisher et al., 1993). Using antibodies to purified SBE I protein from maize endosperm, Baba et al. (1991) isolated a partial-length cDNA encoding the SBE I isoform. This cDNA lacks the entire open reading frame, because no ATG codon was found 5' of the known plastid signal peptide cleavage site. Based on similarity to the rice SBE I-like cDNA (rbel; Baba et al., 1991), it was hypothesized that the SBE I cDNA lacked only two bases of the coding region. As part of our long-term goal to characterize genetic loci encoding maize SBE isoforms, we isolated an apparently full-length SBE I cDNA that includes the entire coding region and 5' and 3' untranslated regions. Based on the