Flavodiiron proteins and NDH-1 cooperate to maintain photosynthetic redox poise in Synechocystis
In oxygenic photosynthetic organisms excluding angiosperms, flavodiiron proteins (FDPs) catalyze light-dependent reduction of O2 to H2O. This alleviates electron pressure on the photosynthetic apparatus and protects it from photodamage. In Synechocystis sp. PCC 6803, four FDP isoforms function as hetero-oligomers of Flv1 and Flv3 and/or Flv2 and Flv4. An alternative electron transport pathway mediated by the NAD(P)H dehydrogenase-like complex (NDH-1) also contributes to redox hemostasis and the photoprotection of photosynthesis. Four NDH-1 types haven been characterized in cyanobacteria: NDH-11 and NDH-12, which function in respiration; and NDH-13 and NDH-14, which function in CO2 uptake. All four types are involved in cyclic electron transport. Along with single FDP mutants ({Delta}flv1 and {Delta}flv3) and the double NDH-1 mutants ({Delta}d1d2, which is deficient in NDH-11,2 and {Delta}d3d4, which is deficient in NDH-13,4), we studied triple mutants lacking either one of Flv1 or Flv3, and NDH-11,2 or NDH-13,4. We show that the presence of either Flv1/3 or NDH-11,2, but not NDH-13,4, is indispensable for survival during changes in growth conditions from high CO2 /moderate light to low CO2 / high light. Our results suggest functional redundancy and crosstalk between FDPs and NDH-11,2 under the studied conditions, and demonstrate that the functions of FDPs and NDH-11,2 are dynamically coordinated for the efficient oxidation of PSI and for photoprotection under variable CO2 and light availability. One sentence summaryFlavodiiron proteins and NDH-1 complex ensure survival of cyanobacterial cells by cooperatively safeguarding the photosynthetic apparatus against excessive reduction