The Two Genes Encoding Starch-Branching Enzymes IIa and IIb Are Differentially Expressed in Barley1

Abstract

The sbeIIa and sbeIIb genes, encoding starch-branching enzyme (SBE) IIa and SBEIIb in barley (Hordeum vulgare L.), have been isolated. The 5 portions of the two genes are strongly divergent, primarily due to the 2064-nucleotide-long intron 2 in sbeIIb. The sequence of this intron shows that it contains a retro-transposonlike element. Expression of sbeIIb but not sbeIIa was found to be endosperm specific. The temporal expression patterns for sbeIIa and sbeIIb were similar and peaked around 12 d after pollination. DNA gel-blot analysis demonstrated that sbeIIa and sbeIIb are both single-copy genes in the barley genome. By fluorescence in situ hybridization, the sbeIIa and sbeIIb genes were mapped to chromosomes 2 and 5, respectively. The cDNA clones for SBEIIa and SBEIIb were isolated and sequenced. The amino acid sequences of SBEIIa and SBEIIb were almost 80% identical. The major structural difference between the two enzymes was the presence of a 94-amino acid N-terminal extension in the SBEIIb precursor. The (␤/ ␣) 8 -barrel topology of the ␣-amylase superfamily and the catalytic residues implicated in branching enzymes are conserved in both barley enzymes.Starch is a mixture of amylose and amylopectin, both of which are Glc polymers. Amylose is a mostly linear polymer of 200 to 2000 ␣-1,4-bonded Glc moieties with rare ␣-1,6 branch points (for reviews, see Martin and Smith, 1995; Ball et al., 1996). Amylopectin is highly ␣-1,6-branched, with a complex structure of 10 6 to 10 8 M r and up to 3 ϫ 10 6 Glc subunits, making it one of the largest biological molecules in nature. In the plant, starch is deposited as starch granules in chloroplasts of photosynthetic tissues or in amyloplasts of endosperm, embryos, tubers, and roots. In most plants, starch consists of 20% to 30% amylose and 70% to 80% amylopectin. In photosynthetic and nonphotosynthetic tissues the Glc moiety of ADP-Glc is incorporated in the growing amylose polymer with the help of starch synthases. The formation of ␣-1,6 linkages in amylopectin is catalyzed by SBEs (EC 2.4.1.18). The final structure of amylopectin is governed by the activities of different SBEs, starch synthases, and a debranching enzyme (Ball et al., 1996).SBEs exist as several isoforms in developing storage tissues of maize, rice, pea (for review, see Martin and Smith, 1995), barley (Sun et al., 1996(Sun et al., , 1997, wheat (Morell et al., 1997), potato (Larsson et al., 1996), and Arabidopsis (Fisher et al., 1996). SBEs can be separated into two major groups based on structural and catalytic properties. One group, referred to as SBE family II or A (Martin and Smith, 1995), comprises SBEII from maize (Fisher et al., 1993; Gao et al., 1997), wheat (Nair et al., 1997), and potato (Larsson et al., 1996), SBE3 from rice (Mizuno et al., 1993), SBEI from pea (Bhattacharyya et al., 1990), and SBE2 from Arabidopsis (Fisher et al., 1996). The other group, SBE family I or B (Martin and Smith, 1995), comprises SBEI from maize (Baba at al., 1991), wheat (Morell et al., 1997), potato (Kossm...

Significance of Light, Sugar, and Amino Acid Supply for Diurnal Gene Regulation in Developing Barley Caryopses

“…7). As reported earlier (Sun et al, 1998;Burton et al, 1999;Radchuk et al, 2009), the transcripts of HvSBEIIb and HvLD were mainly found in the endosperm and are thus likely to be involved in the biosynthesis of storage starch (Table I; Fig. 8).…”

Diurnal oscillation of SBE expression in sorghum endosperm

“…1). This is similar to the situation in barley and might reflect a differential partitioning of the SBEIIA and SBEIIB isoforms (Sun et al, 1998).…”

“…(2003) using 32 P-labelled gene-specific cDNA fragment for sorghum SBEI and SBEIIB , sorghum SBEIIA (this work) and barley SBEIIB (Sun et al, 1998). The membranes were striped of the radioactive probes in a boiling 0.5% (w/v) SDS solution and re-hybridized with 18S rRNA-labeled probes.…”

Starch accumulation, activities of key enzyme and gene expression in starch synthesis of wheat endosperm with different starch contents

“…SBE-IIb plays a specific role in the formation of Achains of the amylopectin cluster (Nishi et al 2001), and the levels of BEIIb activity affect the extent of structural changes in the cluster (Tanaka et al 2004). In monocots, SBE-II is made up of two closely related but discrete gene products (SBE-IIa and SBE-IIb) (Sun et al 1998;Rahman et al 2001;McCue et al 2002). Downregulation of SBE-I activity appears to have minimal effects on starch syntheses and composition in photosynthetic and non photosynthetic tissues (Satoh et al 2003).…”