Chloroplastic NAD(P)H dehydrogenase (NDH) plays a role in cyclic electron flow around photosystem I to produce ATP, especially in adaptation to environmental changes. Although the NDH complex contains 11 subunits that are homologous to NADH:ubiquinone oxidoreductase (complex I; EC 188.8.131.52), recent genetic and biological studies have indicated that NDH also comprises unique subunits. We describe here an in silico approach based on co-expression analysis and phylogenetic profiling that was used to identify 65 genes as potential candidates for NDH subunits. Characterization of 21 Arabidopsis T-DNA insertion mutants among these ndh gene candidates indicated that three novel ndf (NDH-dependent cyclic electron flow) mutants (ndf1, ndf2 and ndf4) had impaired NDH activity as determined by measurement of chlorophyll fluorescence. The amount of NdhH subunit was greatly decreased in these mutants, suggesting that the loss of NDH activity was caused by a defect in accumulation of the NDH complex. In addition, NDF1, NDF2 and NDF4 proteins co-migrated with the NdhH subunit, as shown by blue native electrophoresis. These results strongly suggest that NDF proteins are novel subunits of the NDH complex. Further analysis revealed that the NDF1 and NDF2 proteins were unstable in the mutants lacking hydrophobic subunits of the NDH complex, but were stable in mutants lacking the hydrophilic subunits, suggesting that NDF1 and NDF2 interact with a hydrophobic sub-complex. NDF4 protein was predicted to possess a redox-active iron-sulfur cluster domain that may be involved in the electron transfer.
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