The structure and expression of the wheat starch synthase III gene. Motifsin the expressed gene define the lineage of the starch synthase III gene family

The endosperm of hexaploid wheat (Triticum aestivum [L.]) was shown to cont ain a high molecular weight starch synthase (SS) analogous to the product o f the maize du1 gene, starch synthase LII (SSIII; DU1). cDNA and genomic DN A sequences encoding wheat SSIII were isolated and characterized. The wheat SSIII cDNA is 5,346 bp long and contains an open reading frame that encode s a 1,628-amino acid polypeptide. A putative N-terminal transit peptide, a 436-amino acid C-terminal catalytic domain, and a central 470-amino acid SS III-specific domain containing three regions of repeated amino acid similar ity were identified in the wheat gene. A fourth region between the transit peptide and the SSIII-specific domain contains repeat motifs that are varia ble with respect to motif sequence and repeat number between wheat and maiz e. In dicots, this N-terminal region does not contain repeat motifs and is truncated. The gene encoding wheat SSIII, designated ss3, consists of 16 ex ons extending over 10 kb, and is located on wheat chromosome I. Expression of ss3 mRNA in wheat was detected in leaves, pre-anthesis florets, and from very early to middle stage of endosperm development. The entire N-terminal variable repeat region and the majority of the SSIII-specific domain are e ncoded on a single 2,703-bp exon. A gene encoding a class III SS from the A rabidopsis genome sequencing project shows a strongly conserved exon struct ure to the wheat ss3 gene, with the exception of the N-terminal region. The evolutionary relationships of the genes-encoding monocot and dicot class I II SSs are discussed.