Abstract

The type I NAD(P)H dehydrogenase complex (NDH-1) in cyanobacteria is involved in both respiratory and photosynthetic electron transport processes. NDH-1 is also essential for inorganic carbon transport. It has been postulated that NDH-1-dependent cyclic electron flow around PSI energizes CO₂ uptake. The genome information of Synechocystis sp. PCC6803 has enabled us to provide an integrative view of the CO₂ concentrating mechanism in this organism. In an attempt to dissect the role of the NDH-1 complex, we have constructed single and double mutants of Synechocystis 6803 by disrupting highly homologous ndhD genes in pairs, and have analysed the growth, CO₂ uptake activities, and redox levels of P700 and the plastoquinone pool in these mutants under various conditions. We have also determined the membrane localization of this membrane protein. Our studies have revealed that: (i) mutations in ndh genes lead to inhibition of CO₂ uptake, rather than HCO₃^– uptake; (ii) NDH-1 complexes are localized only in the thylakoid membrane; (iii) there are functionally distinct NDH-1 complexes in Synechocystis #6803. Based on these data, we propose a schematic view of the roles of different NDH-1 complexes in cyanobacteria.