Cyanobacterial and plant NDH-1 subunit OThe proton-pumping NADH:ubiquinone oxidoreductase catalyzes the electron transfer from NADH to ubiquinone linked with proton translocation across the membrane. It is the largest, most complex and least understood of the respiratory chain enzymes and is referred to as Complex I. The subunit composition of the enzyme varies between groups of organisms. Complex I originating from mammalian mitochondria contains 45 different proteins, whereas in bacteria, the corresponding complex NDH-1 consists of 14 different polypeptides. homologs of these 14 proteins are found among subunits of the mitochondrial complex I, and therefore bacterial NDH-1 might be considered a model proton-pumping NADH dehydrogenase with a minimal set of subunits. Escherichia coli NDH-1 readily disintegrates into 3 subcomplexes: a water-soluble NADH dehydrogenase fragment (NuoE, -F, and -G),the connecting fragment (NuoB, -C, -D, and -I), and the membrane fragment (NuoA, -H, -J, -K, -L, -M, -N). In cyanobacteria and their descendants, the chloroplasts of green plants, the subunit composition of NDH-1 remains obscure. The genes for eleven subunits NdhA-NdhK, homologous to the NuoA-NuoD and NuoH-NuoN of the E. coli complex, have been found in the genome of Synechocystis sp. PCC 6803 which has a family of 6 ndhD genes and a family of 3 ndhF genes. Two reported multisubunit complexes, NDH-1L and NDH-1M, represent distinct NDH-1 complexes in the thylakoid membrane of Synechocystis 6803 -cyanobacterium. NDH-1L was shown to be essential for photoheterotrophic cell growth, whereas expression of NDH-1M was a prerequisite for CO2 uptake and played an important role in growth of cells at low CO2. Here we report the subunit composition of these two complexes. Fifteen proteins were discovered in NDH-1L including NdhL, a new component of the membrane fragment, and Ssl1690 (designated as NdhO), a novel peripheral subunit. The three nuclear-encoded subunits NdhM,NdhN and NdhO are vital for the functional integrity of the plastidial complex.