Cloning, expression, and characterization of soluble starch synthase I cDNA from taro (Colocasia esculenta Var. esculenta).

Soluble starch synthase I (SSSI) cDNA was isolated from taro (Colocasia esculenta var. esculenta) by RT-PCR and rapid amplification of cDNA ends reaction. The transcript of this single-copy gene is 2340 bp and encodes 642 amino acids protein containing a putative transit peptide of 54 residues. Recombinant SSSI protein displayed both primer-dependent and primer-independent activities of starch synthase. More SSSI transcript was expressed in taro leaves than in tubers, with no evident expression in petioles; and more transcript and protein were found in tubers of 597 +/- 37 g of fresh weight than in smaller or larger ones. Two forms of SSSI, i.e., 72 and 66 kDa, exist in leaves, and only the 66 kDa form was found in tubers. The taro SSSI, proposed as a novel member, was located only in the soluble fraction of tuber extract, while SSSI from other sources exist in both soluble and granule-bound forms.