Molecular cloning and characterization of granule bound starch synthase I cDNA from a grain amaranth (Amaranthus cruentus l.)

Abstract

A full-length cDNA clone encoding granule-bound starch synthase I (GBSSI = Waxy gene) from grain amaranth (Amaranthus cruentus L.) perisperm was isolated and characterized. Segregation of amylose content in F2 population suggested that the amylose content of A cruentus is controlled by a single gene, Waxy (GBSSI). cDNA clone of this gene is 2076 bp in length and contains an open reading frame of 1821 bp corresponding to a polypeptide of 606 amino acids residues, including a transit peptide of 77 amino acids. Comparison of the cDNA and genomic sequences (3492 bp) suggested that the amaranth GBSSI gene has 12 introns, of which exons 1-13 contributed to the coding sequence. The mature protein shares 70.2-75.3% sequence identity with GBSSI of dicots and about 64.0-67.8% identity with those of monocots. This protein contains the conserved motif KTGGL found in other GBSSI proteins, which has been implicated as the active site in glycogen synthase. Sequence analysis predicted that GBSSI of amaranth has a transit peptide of 77 amino acids including FIR↓S, which is different cleavage site that of the other dicot species. These results will provide more useful information for understanding the structure/function relationship of this protein from amaranths perisperm.