Biocatalytic role of potato starch synthase III for α-glucan biosynthesis in Synechocystis sp. PCC6803 mutants.

A potato starch synthase III (PSSIII) was expressed in the Synechocystis mutants deficient in either glycogen synthase I (M1) or II (M2) to replenish α-(1,4) linkage synthesizing activity, resulting in new mutants, PM1 and PM2, respectively. These mutants were applied to study the role of exogenous plant starch synthase for starch/glycogen biosynthesis mechanism established in the cyanobacteria. The remaining glycogen synthase genes in PM1 and PM2 were further disrupted to make the mutants PM12 and PM21 which contained PSSIII as the sole glycogen/starch synthase. Among wild type and mutants, there were no significant differences in the amount of α-glucan produced. All the mutants harboring active PSSIII produced α-glucans with relatively much shorter and less longer α-1,4 chains than wild-type glycogen, which was exactly in accordance with the increase in glycogen branching enzyme activity. In fact, α-glucan structure of PM1 was very similar to those of PM12 and PM21, and PM2 had more intermediate chains than M2. This result suggests PSSIII may have distributive elongation property during α-glucan synthesis. In conclusion, the Synechocystis as an expression model system of plant enzymes can be applied to determine the role of starch synthesizing enzymes and their association during α-glucan synthesis.