Disruption of Polyhydroxybutyrate Synthesis Redirects Carbon Flow towards Glycogen Synthesis in Synechocystis sp. PCC 6803 Overexpressing glgC/glgA

The photoautotrophic Synechocystis sp. PCC 6803 (hereafter Synechocystis) is known for its α-polyglucan (glycogen) synthesis to serve as a carbon storage compound. In this study, the glgC- and glgA-overexpressing Synechocystis strain with the disruption of polyhydroxybutyrate (PHB) synthesis (▴GCAX-ΔBK) showed an increased glycogen production. This engineered strain had a high glycogen content of 38.3% (g g–¹ dry cell weight) as compared with 27.4% in the phaA knockout strain (ΔBK) and 34.8% in the glgC/glgA-overexpressing strain (▴GCAX) after 20 d growth. Under nitrogen-deprived growth conditions for 3 d, the ▴GCAX-ΔBK strain showed a further increase in glycogen content from 27.0% to 36.0%. Furthermore, the engineered strains grown under ionic, osmotic or oxidative stress conditions had an increase of glycogen accumulation, whereas no increase was observed in the wild type. The maximum glycogen content was 54.0% in the ▴GCAX-ΔBK strain treated with 3 mM H₂O₂. The overall results indicated that in the absence of PHB synthesis, Synechocystis cells redirected the carbon flow towards the synthesis of glycogen as an alternative physiological responsive compound especially under stress conditions.