An Expression of the Sweet Potato GBSSI Gene in Escherichia coli

A starch synthase cDNA clone (spss67) was isolated from tuberous roots of sweet potato. This clone contained an open reading frame corresponding to a protein of 608 amino acid residues, which also included three consensus regions of starch synthases. Amino acid sequence analyses showed that the protein product of spss67 shared a high degree of homology with other plant granule-bound starch synthase I (GBSSI). Comparisons of N-terminal amino acid sequence of spss67 cDNA protein product with that of the native protein isolated from sweet potato indicated that the protein contained a signal peptide of 77 amino acids, and the transit peptide had hydrophobic regions in the initiation and end of peptide that was also present in GBSSI of other plant species. In addition, the protein product of spss67 expressed in E. coli could be recognized by anti-GBSSI antibody of potato. Moreover, this protein exhibited a high activity of starch synthase when amylose was used as a primer, but the activity was not significant when the primer was replaced by amylopectin or glycogen. Studies based on the sequence analyses, western blotting and priming activity indicated that the product of spss67 clone was a GBSSI