Identification of an Alternative Glycogen Synthesis Pathway Independent of Glycogen Synthases in the Cyanobacterium Synechocystis Sp. PCC 6803.

In cyanobacteria, glycogen is the primary carbon storage polysaccharide. It is synthesised via the canonical GlgC-GlgA pathway, which involves the enzymes ADP-glucose pyrophosphorylase (AGP) and glycogen synthase (GlgA). The Synechocystis sp. PCC 6803 mutant lacking glycogen synthases (ΔglgA) is not viable due to toxic accumulation of ADP-glucose, but can survive under saline conditions by redirecting this precursor to the synthesis of the osmolyte glucosylglycerol (GG). In this study, we discovered that this viable ΔglgA mutant accumulates substantial amounts of a high-molecular weight intracellular polysaccharide. Analysis by transmission electron microscopy, indicated that this polymer forms intracellular granules visually similar to the glycogen in the wild type. Physiological studies confirmed that this polysaccharide is functionally analogous to glycogen, thus accumulating during nitrogen starvation and being mobilised by glycogen phosphorylase. The synthesis of this glycogen-like polysaccharide requires the enzymes AGP and glucosylglycerol-phosphate synthase (GgpS), indicating a direct link to the osmolyte production pathway. Furthermore, exogenous GG supplementation partially restored polysaccharide accumulation in a mutant lacking both GlgAs and GgpS enzymes, strongly suggesting that GG, or a closely related derivative, serves as an essential precursor for this GlgA-independent synthesis of glycogen. The specific enzyme responsible for catalysing this novel glucan elongation reaction remains to be identified. This work reveals a previously unknown biosynthetic route of glycogen synthesis, highlighting the metabolic plasticity of cyanobacteria by functionally integrating an intermediate from osmolyte metabolism into cellular carbon storage pathways.