Abstract
BackgroundFlavodiiron proteins (FDPs) comprise a group of modular enzymes that function in oxygen and nitric oxide detoxification in Bacteria and Archaea. The FDPs in cyanobacteria have an extra domain as compared to major prokaryotic enzymes. The physiological role of cyanobacteria FDPs is mostly unknown. Of the four putative flavodiiron proteins (Flv1–4) in the cyanobacterium Synechocystis sp. PCC 6803, a physiological function in Mehler reaction has been suggested for Flv1 and Flv3.Principal FindingsWe demonstrate a novel and crucial function for Flv2 and Flv4 in photoprotection of photosystem II (PSII) in Synechocystis. It is shown that the expression of Flv2 and Flv4 is high under air level of CO2 and negligible at elevated CO2. Moreover, the rate of accumulation of flv2 and flv4 transcripts upon shift of cells from high to low CO2 is strongly dependent on light intensity. Characterization of FDP inactivation mutants of Synechocystis revealed a specific decline in PSII centers and impaired translation of the D1 protein in Δflv2 and Δflv4 when grown at air level CO2 whereas at high CO2 the Flvs were dispensable. Δflv2 and Δflv4 were also more susceptible to high light induced inhibition of PSII than WT or Δflv1 and Δflv3.SignificanceAnalysis of published sequences revealed the presence of cyanobacteria-like FDPs also in some oxygenic photosynthetic eukaryotes like green algae, mosses and lycophytes. Our data provide evidence that Flv2 and Flv4 have an important role in photoprotection of water-splitting PSII against oxidative stress when the cells are acclimated to air level CO2. It is conceivable that the function of FDPs has changed during evolution from protection against oxygen in anaerobic microbes to protection against reactive oxygen species thus making the sustainable function of oxygen evolving PSII possible. Higher plants lack FDPs and distinctly different mechanisms have evolved for photoprotection of PSII.
Highlights
The flavodiiron proteins (FDPs) are a large family of soluble enzymes found in strict or facultative anaerobes among Bacteria and Archaea as well as in some protozoan pathogens, reviewed in [1]
Flavodiiron proteins are abbreviated as FDPs in recent reviews [1,11], we have here adapted nomenclature of Synechocystis proteins according to Helman et al [10] as Flv proteins (Flv1, Flv2, Flv3 and Flv4)
As photoautotrophic inhabitants of aquatic environments, cyanobacteria are challenged by fluctuation of light and deficiency of inorganic carbon in their natural environments
Summary
The flavodiiron proteins (FDPs) are a large family of soluble enzymes found in strict or facultative anaerobes among Bacteria and Archaea as well as in some protozoan pathogens, reviewed in [1]. Despite the structural similarity of FDPs, studies on these proteins from various organisms have revealed significantly different redox potentials [4,5], which indicate that the members of this family may interact with distinct redox partners and participate in different cellular processes. The oxygen reductase activity of FDPs functions in preventing oxygen toxicity in some obligatory anaerobes when cells are transiently exposed to oxygen [6,7]. Other FDPs like Escherichia coli (E. coli) flavorubredoxin prefer NO as a substrate, functioning in NO detoxification [8]. Flavodiiron proteins (FDPs) comprise a group of modular enzymes that function in oxygen and nitric oxide detoxification in Bacteria and Archaea. PCC 6803, a physiological function in Mehler reaction has been suggested for Flv and Flv
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