Abstract
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.
Highlights
A-type flavodiiron proteins (Flvs or Flavodiiron proteins (FDPs)) were originally identified in strict and facultative anaerobes among Bacteria, Archaea and Protozoa and were considered to function in O2 and/or NO detoxification (Wasserfallen et al, 1998; Gonçalves et al., 2011; Folgosa et al, 2018)
The structures of FDPs in anaerobic prokaryotes and eukaryotic protozoa have been resolved as homooligomers arranged in a “head-to-tail” configuration, so that the diiron center of one monomer and the flavin mononucleotide (FMN) of the other monomer are in close proximity to each other, which ensures rapid electron transfer between the two cofactors
Flv4 both in LC and high Ci (> 1% CO2 in air, HC) conditions (Bersanini et al, 2014), demonstrated substantially higher O2 photoreduction rates compared to the respective
Summary
A-type flavodiiron proteins (Flvs or FDPs) were originally identified in strict and facultative anaerobes among Bacteria, Archaea and Protozoa and were considered to function in O2 and/or NO detoxification (Wasserfallen et al, 1998; Gonçalves et al., 2011; Folgosa et al, 2018). The structures of FDPs in anaerobic prokaryotes and eukaryotic protozoa have been resolved as homooligomers (dimer or tetramer comprised of two dimers) arranged in a “head-to-tail” configuration, so that the diiron center of one monomer and the FMN of the other monomer are in close proximity to each other, which ensures rapid electron transfer between the two cofactors. C-type FDPs, specific to oxygenic photosynthetic organisms, hold an additional flavinreductase-like domain, coupled with extra cofactors (Romão et al, 2016; Folgosa et al, 2018). PCC 6803 (hereafter, Synechocystis) possesses four genes encoding FDPs: sll1521 (Flv1), sll0219 (Flv2), sll0550 (Flv3) and sll0217 (Flv4)
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