Type 2 NADH dehydrogenases in the cyanobacterium Synechocystis sp strain PCC 6803 are involved in regulation rather than respiration

Analysis of the genome of Synechocystis sp, strain PCC 6803 reveals three o pen reading frames (slr0851, slr1743, and sll1484) that mag code for type 2 NAD(P)N dehydrogenases (NDH-2). The sequence similarity between the transl ated open reading frames and NDH-2s from other organisms is low, generally not exceeding 30% identity. However, NAD(P)H and flavin adenine dinucleotid e binding motifs are conserved in all three putative NDH-2s in Synechocysti s sp, strain PCC 6803, The three open reading frames H ere cloned, and dele tion constructs were made for each. An expression construct containing one of the three open reading frames, slr1743, was able to functionally complem ent an Escherichia coli mutant lacking both NDH-1s and NDH-2s, Therefore, s lr0851. slr1743, and sll1484 have been designated ndbA, ndbB, and ndbC, res pectively. Strains that lacked one or more of the ndb genes were created in wild-type and photosystem (PS) I-less backgrounds. Deletion of ndb genes l ed to small changes in photoautotrophic growth rates and respiratory activi ties. Electron transfer rates into the plastoquinone pool in thylakoids in darkness were consistent with the presence of a small amount of NDH-2 activ ity in thylakoids. No difference was observed between wild-type and the Ndb -less strains in the banding patterns seen on native gels when stained for either NADH or NADPH dehydrogenase activity, indicating that the Ndb protei ns do not accumulate to high levels. A striking phenotype of the PS I-less background strains lacking one or more of the NDH-2s is that they were able to grow at high light intensities that were lethal to the control strain b ut they retained normal PS II activity. We suggest that the Ndb proteins in Synechocystis sp. strain PCC 6803 are redox sensors and that they play a r egulatory role responding to the redox state of the plastoquinone pool.