Overexpressing Photosynthetic Carbon Flux Control Enzymes in Synechocystis PCC 6803

It is environmentally sustainable to convert inorganic carbon directly into desired compounds with photoautotrophic organisms, like higher plants, algae, and cyanobacteria. The Calvin cycle is the primary carbon fixation pathway in photoautotrophic organisms. In higher plants, Ribulose-1,5-biphosphate carboxylation (RuBP) carboxylation and RuBP regeneration together control the carbon flux in this process based on experimental data and dynamic mimic data. Rubilose-1,5-carboxylase/oxygenase (RuBisCO) is the enzyme catalyzing RuBP carboxylation. For RuBP regeneration, Seduheptoluse-1,7-biphosphatase (SBPase), aldolase (FBA) and transketolase (TK) are the three main enzymes controlling photosynthetic rate. Data from cyanobacteria is still missing. We succeeded overexpressing native RuBisCO, SBPase (fructose-1,6-/sedoheptoluse-1,7-biphosphatase, FBP/SBPase), FBA and TK separately in Synechocystis PCC 6803. The engineering strains showed different phenotypes compared to control strain under 100 μmol photons m^-2 s^-1 light conditions in shaking flasks. Growth rate, maximal in vivo oxygen evolution rate and dry weight increased in RuBisCO, FBP/SBPase, FBA overexpressed strains. TK overexpressed strain showed chlorotic phenotype and increased dry weight. These results indicate these four enzymes are key enzymes in Calvin cycle in Synechocystis PCC 6803 and subsequently are potential targets to increase photosynthetic rates and yields.