Characterization of cyanobacterial glycogen isolated from the wild type and from a mutant lacking of branching enzyme Original Research Article

Cyanobacteria produce glycogen as their primary form of carbohydrate storage. The genomic DNA sequence of Synechocystis sp. PCC6803 indicates that this strain encodes one glycogen-branching enzyme (GBE) and two isoforms of glycogen synthase (GS). To confirm the putative GBE and to demonstrate the presence of only one GBE gene, we generated a mutant lacking the putative GBE gene, sll0158, by replacing it with a kanamycin resistance gene through homologous recombination. GBE in sll0158− mutant was eliminated; the mutant strain produced less glucan, equivalent to 48% of that produced by the wild type. In contrast to the wild-type strain that had 74% of the glucan being water-soluble, the mutant had only 14% of the glucan water-soluble. Molecular structures of glucans produced by the mutant and the wild type were characterized by using high-performance size-exclusion and anion-exchange chromatography. The glycogen produced by the wild type displayed a molecular mass of 6.6×107 daltons (degree of polymerization (DP) 40 700) and 10% branch linkages, and the α-d-glucan produced by the mutant displayed a molecular mass of 4.7–5.6×103 daltons (DP 29–35) with slight branch linkages. The results indicated that sll0158 was the major functional GBE gene in Synechocystis sp. PCC6803.