Cloning and functional analysis of a cDNA encoding a starch synthase from potato (Solanum tuberosum L,) that is predominantly expressed in leaf tissue

Three isoforms of starch synthase (SS) were shown to be present in soluble potato tuber extracts by activity staining after native gel electrophoresis . A cDNA encoding SSI from rice was used as a probe to clone a correspondin g cDNA from potato. The deduced amino acid sequence identified the protein as an SS from potato with an M-r of 70.6 kDa for the immature enzyme includ ing its transit peptide. This novel isoform was designated SSI. An analysis of the expression pattern of the gene indicated that SSI is predominantly expressed in sink and source leaves, and, to a lower extent in tubers. In s everal independent transgenic potato lines, where the expression of SSI was repressed using the antisense approach, the activity of a specific S'S iso form was reduced to non-detectable levels as determined through activity st aining after native gel electrophoresis. The reduction in the amount of thi s isoform of SS did not lead to any detectable changes in starch structure! probably due to the fact that this isoform only represents a minor activit y in potato tubers.