Starch branching enzymes (SBE) which catalyse the formation of α-1,6-glucan linkages are of crucial importance for the quantity and quality of starch synthesized in plants. In maize (Zea mays L.), three SBE isoforms (SBEI, IIa and IIb) have been identified and shown to exhibit differential expression patterns. As a first step toward understanding the regulatory mechanisms controlling their expression, we isolated and sequenced a maize genomic DNA (-2190 to +5929) which contains the entire coding region of SBEI (Sbe1) as well as 5'-and 3'-flanking sequences. Using this clone, we established a complete genomic organization of the maize Sbe1 gene. The transcribed region consists of 14 exons and 13 introns, distributed over 5.7 kb. A consensus TATA-box and a G-box containing a perfect palindromic sequence, CCACGTGG, were found in the 5'-flanking region. Genomic Southern blot analysis indicated that two Sbe1 genes with divergent 5'-flanking sequences exist in the maize genome, suggesting the possibility that they are differentially regulated. A chimeric construct containing the 5'-flanking region of Sbe1 (-2190 to +27) fused to the β-glucuronidase gene (pKG101) showed promoter activity after it was introduced into maize endosperm suspension cells by particle bombardment.
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