Flavodiiron proteins 1–to-4 function in versatile combinations in O₂ photoreduction in cyanobacteria

Flavodiiron proteins (FDPs) constitute a group of modular enzymes widespread in Bacteria, Archaea and Eukarya. Synechocystis sp. PCC 6803 has four FDPs (Flv1-4), which are essential for the photoprotection of photosynthesis. A direct comparison of light-induced O₂ reduction (Mehler-like reaction) under high (3% CO₂, HC) and low (air level CO₂, LC) inorganic carbon conditions demonstrated that the Flv1/Flv3 heterodimer is solely responsible for an efficient steady-state O₂ photoreduction under HC, with flv2 and flv4 expression strongly down-regulated. Conversely, under LC conditions, Flv1/Flv3 acts only as a transient electron sink, due to the competing withdrawal of electrons by the highly induced NDH-1 complex. Further, in vivo evidence is provided indicating that Flv2/Flv4 contributes to the Mehler-like reaction when naturally expressed under LC conditions, or, when artificially overexpressed under HC. The O₂ photoreduction driven by Flv2/Flv4 occurs down-stream of PSI in a coordinated manner with Flv1/ Flv3 and supports slow and steady-state O₂ photoreduction.