Skip to main page content
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
, 110 (10), 4111-6

Flavodiiron Proteins Flv1 and Flv3 Enable Cyanobacterial Growth and Photosynthesis Under Fluctuating Light

Affiliations

Flavodiiron Proteins Flv1 and Flv3 Enable Cyanobacterial Growth and Photosynthesis Under Fluctuating Light

Yagut Allahverdiyeva et al. Proc Natl Acad Sci U S A.

Abstract

Cyanobacterial flavodiiron proteins (FDPs; A-type flavoprotein, Flv) comprise, besides the β-lactamase-like and flavodoxin domains typical for all FDPs, an extra NAD(P)H:flavin oxidoreductase module and thus differ from FDPs in other Bacteria and Archaea. Synechocystis sp. PCC 6803 has four genes encoding the FDPs. Flv1 and Flv3 function as an NAD(P)H:oxygen oxidoreductase, donating electrons directly to O2 without production of reactive oxygen species. Here we show that the Flv1 and Flv3 proteins are crucial for cyanobacteria under fluctuating light, a typical light condition in aquatic environments. Under constant-light conditions, regardless of light intensity, the Flv1 and Flv3 proteins are dispensable. In contrast, under fluctuating light conditions, the growth and photosynthesis of the Δflv1(A) and/or Δflv3(A) mutants of Synechocystis sp. PCC 6803 and Anabaena sp. PCC 7120 become arrested, resulting in cell death in the most severe cases. This reaction is mainly caused by malfunction of photosystem I and oxidative damage induced by reactive oxygen species generated during abrupt short-term increases in light intensity. Unlike higher plants that lack the FDPs and use the Proton Gradient Regulation 5 to safeguard photosystem I, the cyanobacterial homolog of Proton Gradient Regulation 5 is shown not to be crucial for growth under fluctuating light. Instead, the unique Flv1/Flv3 heterodimer maintains the redox balance of the electron transfer chain in cyanobacteria and provides protection for photosystem I under fluctuating growth light. Evolution of unique cyanobacterial FDPs is discussed as a prerequisite for the development of oxygenic photosynthesis.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Growth phenotype of Synechocystis WT and mutant cells. (A) Cells were grown under FL 20/500 or 50/500 regime for 4 d. (B) Cells were grown under FL 20/500 regime for 4 d followed by adjustment of OD750 to 0.1 and shifting to constant light. (C) Cells were grown under FL 50/500 regime for 4 d followed by adjustment of OD750 to 0.1 and shifting to constant light. (D) Cells were grown under FL 20/500 regime for 8 d followed by adjustment of OD750 to 0.1 and shifting to constant light. Mean ± SD; n = 4. WT, ■; Δflv1::flv1 complementation strain, ★; mutant strains Δflv1, ○; Δflv3, ∇; Δflv1flv3, ◇.
Fig. 2.
Fig. 2.
Real-time O2 (A and C) and CO2 (B and D) exchange measurements in Synechocystis WT and Δflv1flv3 cells. The cells were grown under constant light of 50 μmol photons⋅m−2⋅s−1 and then set at OD750 = 0.4–0.5 and shifted to FL 20/500 for 2–3 d before the measurements. Actinic white light intensity of 20 μmol photons⋅m−2⋅s−1 was applied to the dark-adapted cells, and ∼30-s high-light pulses at an intensity of 500 μmol photons⋅m−2⋅s−1 was turned on (up arrows) and off (down arrows) every 4 min. Cumulative O2 evolution (dotted line), O2 uptake (solid line), and net photosynthesis (dashed line) were calculated from 18O2/16O2 exchange measurements.
Fig. 3.
Fig. 3.
Effect of KCN on O2 uptake in the WT and ∆flv1/flv3 cells. O2 uptake was recorded from the WT (black line) and ∆flv1/flv3 cells (gray line) during 10-min darkness and upon exposure to FL in the absence (solid line) and in the presence (dashed line) of 1 mM KCN. Up and down arrows show on and off high-light pulses, respectively.
Fig. 4.
Fig. 4.
Flash-induced increase in variable fluorescence and its subsequent decay in the WT and ∆flv1/∆flv3 cells in the absence and in the presence of DCMU (inset). WT (■) and Δflv1flv3 (∇) were grown under constant-light conditions of 50 μmol photons⋅m−2⋅s−1, then set at OD750 = 0.4–0.5 and shifted to FL 50/500 for 3 d before the measurements. r.u., relative units.
Fig. 5.
Fig. 5.
PSII and PSI properties of the WT and ∆flv1/∆flv3 cells grown under FL. (A, B, and D) Chl fluorescence (gray line) and P700 (black line) were monitored simultaneously in WT (A) and ∆flv1/∆flv3 in the absence (B) and presence (D) of MV. (C) Acceptor-side limitation of PSI was calculated from WT (■) and Δflv1flv3 (△) in the absence of MV. The cells were grown under constant light, then set at OD750 = 0.4–0.5 and shifted to FL 50/500 for 2–3 d before the measurements. Low background actinic light (58 μmol photons⋅m−2⋅s−1; gray arrow) and high light (530 μmol photons⋅m−2⋅s−1; black arrow) was switched on and off (up and down arrow, respectively), mimicking FL. r.u., relative units.
Fig. 6.
Fig. 6.
PSII and PSI properties of the WT and ∆flv1/∆flv3 cells grown under constant light. (A and B) Chl fluorescence (gray line) and absorbance changes of P700 (black line) were recorded simultaneously in WT (A) and ∆flv1/∆flv3 (B). (C) Oxidation of 700 during low- to high-light transitions in ∆flv1/∆flv3. (D) Acceptor-side limitation of PSI in WT (■) and Δflv1flv3 (△). The cells were grown under constant light, then set at OD750 = 0.4–0.5 and shifted to FL 50/500 for 2–3 d before the measurements. Low-background actinic light (58 μmol photons⋅m−2⋅s−1; gray arrow) and high light (530 μmol photons⋅m−2⋅s−1; black arrow) were switched on and off (up and down arrow, respectively), mimicking FL. r.u., relative units.
Fig. 7.
Fig. 7.
Immunoblots demonstrating the relative amounts of proteins in the WT; Δflv1, Δflv3, and Δflv1flv3 mutants; and complementation strain. (A) Protein content of the WT and mutant strains grown 4 d under FL 50/500 regime was analyzed with corresponding antibodies. (B) Changes in the protein content of WT and the Δflv1flv3 mutant upon shift from constant-growth light (day 0) to FL 50/500 for 1, 3, and 7 d. Proteins from total cell extracts (10 µg in each well) were separated by SDS/PAGE, and immunoblotting was performed by using specific antibodies. Flv1 and Flv3 proteins were detected from soluble samples (30 µg in each well).

Similar articles

See all similar articles

Cited by 65 PubMed Central articles

See all "Cited by" articles

Publication types

MeSH terms

LinkOut - more resources

Feedback