Transgenic potato tubers accumulate high levels of 1-kestose and nystose: functional identification of a sucrose sucrose 1-fructosyltransferase of artichoke (Cynara scolymus) blossom discs
1997
Hellwege, E.M. | Gritscher, D. | Willmitzer, L. | Heyer, A.G.
By screening a cDNA library of artichoke (Cynara scolymus) blossom discs for fructosyltransferases, we isolated a clone designated Cy21. The deduced amino acid sequence shows homology to acid beta-fructosyl hydrolases and to the sucrose-fructan 6-fructosyltransferase (6-SFT) of barley. Transiently expressed in Nicotiana tabacum protoplasts, the Cy21 gene-product synthesized 1-kestose, indicating that Cy21 codes for a sucrose sucrose 1-fructosyltransferase (1-SST). The enzyme worked at physiologically relevant sucrose concentrations (25 mM sucrose). In the protoplast system, 1-kestose seemed to be the only fructan product of the 1-SST. The enzyme activity was not affected by pyridoxal-HCl, an inhibitor of both the beta-fructosyl hydrolase and the fructosyltransferase activity of invertases. The fructosyltransferase activity of the Cy21 gene-product, however, could be inhibited by Zn(2+), Ag(+) and Cu(2+) ions. In artichoke plants the Cy21 transcript was highly abundant in primary roots and blossom discs. Transgenic potato tubers expressing Cy21 contain high levels of 1-kestose along with nystose and traces of fructosyl-nystose, supporting the conclusion that the Cy21 clone encodes a sucrose sucrose 1-fructosyltransferase.
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