Starch branching enzymes in sorghum (Sorghum bicolor) and barley (Hordeum vulgare): Comparative analyses of enzyme structure and gene expression

A genomic; clone for starch branching enzyme (SBE) II b was isolated from a sorghum bacterial artificial chromosome (BAC) library. The promoter and 5' flanking sequence, the first four exons and introns as well as the last exon and the 3' untranslated region were sequenced. The tentative transcription start site of sorghum sbellb was mapped based on alignment with the maize sbellb gene. The exon-intron structure of the 5' portion of sorghum sbellb was similar to that of maize sbellb but differed from that of barley sbellb. Specifically, the intronic Bbl element involved in the endosperm specific expression of barley sbellb was lacking in the sorghum gene. A c;DNA clone for sorghum sbellb was reverse PCR amplified and found to encode an 803 amino acids long protein. The amino acid sequence of sorghum SBEIIb was compared to that of sorghum SBEI and corresponding enzymes in barley. The overall identity in amino acid sequence was 54 % in the central portion of the enzymes. A major difference between the SBEII and SBEI isoforms was a 67 amino acids-long C-terminal extension in the SBEls. The spatial and temporal expression patterns of sorghum sbellb was determined and compared with those of the sorghum gene for SBEI and the barley genes for SBEIIB and SBEI. All four genes exhibited a seed specific expression. However, while barley sbeIIb and sbeI transcripts were detected exclusively in the endosperm, the sorghum genes were expressed also in the embryo. The activity of sorghum sbellb and sbeI exhibited a late onset, with a peak of transcription at around 22 days after pollination. This is similar to the pattern of barley sbel but different from that of barley sbeIIb, which showed a peak of transcription at 12 days after pollination.