13C-metabolic flux analysis of respiratory chain disrupted strain ΔndhF1 of Synechocystis sp. PCC 6803.

Cyanobacteria are advantageous hosts for industrial applications toward achieving sustainable society due to their unique and superior properties such as atmospheric CO₂ fixation via photosynthesis. However, cyanobacterial productivities tend to be weak compared to heterotrophic microbes. To enhance them, it is necessary to understand the fundamental metabolic mechanisms unique to cyanobacteria. In cyanobacteria, NADPH and ATP regenerated by linear and cyclic electron transfers using light energy are consumed by CO₂ fixation in a central metabolic pathway. The previous study demonstrated that the strain deleted a part of respiratory chain complex (ΔndhF1) perturbed NADPH levels and photosynthetic activity in Synechocystis sp. PCC 6803. It is expected that disruption of ndhF1 would result in a decrease in the function of cyclic electron transfer, which controls the ATP/NAD(P)H production ratio properly. In this study, we evaluated the effects of ndhF1 deletion on central metabolism and photosynthesis by ¹³C-metabolic flux analysis. As results of culturing the control and ΔndhF1 strains in a medium containing [1,2-¹³C] glucose and estimating the flux distribution, CO₂ fixation rate by RuBisCO was decreased to be less than half in the ΔndhF1 strain. In addition, the regeneration rate of NAD(P)H and ATP by the photosystem, which can be estimated from the flux distribution, also decreased to be less than half in the ΔndhF1 strain, whereas no significant difference was observed in ATP/NAD(P)H production ratio between the control and the ΔndhF1 strains. Our result suggests that the ratio of utilization of cyclic electron transfer is not reduced in the ΔndhF1 strain unexpectedly.