A clickable photosystem I, ferredoxin, and ferredoxin NADP+ reductase fusion system for light-driven NADPH regeneration

Photosynthetic organisms like plants, algae, and cyanobacteria use light for the regeneration of dihydronicotinamide dinucleotide phosphate (NADPH). The process starts with the light-driven oxidation of water by photosystem II (PSII) and the released electrons are transferred via the cytochrome b 6 f complex towards photosystem I (PSI). This membrane protein complex is responsible for the light-driven reduction of the soluble electron mediator ferredoxin (Fd), which passes the electrons to ferredoxin NADP+ oxidoreductase (FNR). Finally, NADPH is regenerated by FNR at the end of the electron transfer chain. In this study, we established a clickable fusion system for in vitro NADPH regeneration with PSI-Fd and PSI-Fd-FNR, respectively. For this, we fused immunity protein 7 (Im7) to the C-terminus of the PSI-PsaE subunit in the cyanobacterium Synechocystis sp. PCC 6803. Furthermore, colicin DNase E7 (E7) fusion chimeras of Fd and FNR with varying linker domains were expressed in E. coli . Isolated Im7-PSI was coupled with the E7-Fd or E7-Fd-FNR fusion proteins through high-affinity binding of the E7/Im7 protein pair. The corresponding complexes were tested for NADPH regeneration capacity in comparison to the free protein systems demonstrating the general applicability of the strategy. ### Competing Interest Statement The authors have declared no competing interest.