Functional redundancy between flavodiiron proteins and NDH-1 in Synechocystis sp. PCC 6803.

In oxygenic photosynthetic organisms, excluding angiosperms, flavodiiron proteins (FDPs) catalyze light-dependent reduction of O to H O. 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 have been characterized in cyanobacteria: NDH-1 and NDH-1 , which function in respiration; and NDH-1 and NDH-1 , which function in CO uptake. All four types are involved in cyclic electron transport. Along with single FDP mutants (∆flv1 and Δflv3) and the double NDH-1 mutants (∆d1d2, which is deficient in NDH-1 and ∆d3d4, which is deficient in NDH-1 ), we studied triple mutants lacking one of Flv1 or Flv3, and NDH-1 or NDH-1 . We show that the presence of either Flv1/3 or NDH-1 , but not NDH-1 , is indispensable for survival during changes in growth conditions from high CO /moderate light to low CO /high light. Our results show functional redundancy between FDPs and NDH-1 under the studied conditions. We suggest that ferredoxin probably functions as a primary electron donor to both Flv1/3 and NDH-1 , allowing their functions to be dynamically coordinated for efficient oxidation of photosystem I and for photoprotection under variable CO and light availability.