THE 2 GENES ENCODING STARCH-BRANCHING ENZYMES IIA AND IIB ARE DIFFERENTIALLY EXPRESSED IN BARLEY

The sbeIIa and sbeIIb genes, encoding starch-branching enzyme (SBE) II a and SBEIIb in barley (Hordeum vulgare L.), have been isolated. The 5 ' portions of the two genes are strongly divergent, primarily due to t he 2064-nucleotide-long intron 2 in sbeIIb. The sequence of this intro n shows that it contains a retro-transposonlike element. Expression of sbeIIb but not sbeIIa was found to be endosperm specific. The tempora l expression patterns for sbeIIa and sheIIb were similar and peaked ar ound 12 d after pollination. DNA gel-blot analysis demonstrated that s beIIa and sbeIIb are both single-copy genes in the barley genome. By f luorescence in situ hybridization, the sbeIIa and sbeIIb genes were ma pped to chromosomes 2 and 5, respectively. The cDNA clones for SBEIIa and SBEIIb were isolated and sequenced. The amino acid sequences of SB EIIa and SBEIIb were almost 80% identical. The major structural differ ence between the two enzymes was the presence of a 94-amino acid N-ter minal extension in the SBEIIb precursor. The (beta/alpha)(8)-barrel to pology of the alpha-amylase superfamily and the catalytic residues imp licated in branching enzymes are conserved in both barley enzymes.