Flv1-4 proteins function in versatile combinations in O2 photoreduction in cyanobacteria

Flavodiiron proteins (FDPs) constitute a group of modular enzymes widespread in all life Domains. Synechocystis sp. PCC 6803 has four FDPs (Flv1-4) essential for photoprotection of photosynthesis. A direct comparison of the Mehler-like reaction (O2 photoreduction) in high Ci (3% CO2, HC) and low Ci (air level CO2, LC) acclimated cells demonstrated that the Flv1/Flv3 heterodimer is 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 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.