SummaryIn order to increase the branching degree of potato tuber starch, the gene encoding branching enzyme (glgB) of Escherichia coil was expressed in the amylose-free potato mutant. The E. coil glgB was cloned in the binary vector pBIN19 under the transcriptional control of the potato Granule Bound Starch Synthase (GBSS) promoter and transitpeptide sequence. The E. coil glgB was cloned behind the two N-terminal amino acids of the GBSS mature protein, creating a chimeric protein. Transgenic plants were obtained which expressed the E. coil branching enzyme as was shown by the presence of mRNA and protein in the tubers. Correctly processed protein was found both in the soluble and starch granule bound protein fraction. Analysis of the starch showed an increase in the branching degree (DE) of up to 25% more branchpoints. The increase in the number of branchpoints was due to the presence of more short chains, with a degree of polymerization (DP) of 16 glucose-residues or less in the amylopectin. Changes in other characteristics of the starch, such as average chain length (CL) and ;~ax, indicated a more branched structure for starch of transformed plants as well.
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