The Rnf complex is a Na+ coupled respiratory enzyme in a fermenting bacterium, Thermotoga maritima

rnfgenes are widespread in bacteria and biochemical and genetic data are in line with the hypothesis that they encode a membrane-bound enzyme that oxidizes reduced ferredoxin and reduces NAD and vice versa, coupled to ion transport across the cytoplasmic membrane. The Rnf complex is of critical importance in many bacteria for energy conservation but also for reverse electron transport to drive ferredoxin reduction. However, the enzyme has never been purified and thus, ion transport could not be demonstrated yet. Here, we have purified the Rnf complex from the anaerobic, fermenting thermophilic bacteriumThermotoga maritimaand show that is a primary Na(+)pump. These studies provide the proof that the Rnf complex is indeed an ion (Na+) translocating, respiratory enzyme. Together with a Na+-F(1)F(O)ATP synthase it builds a simple, two-limb respiratory chain inT. maritima. The physiological role of electron transport phosphorylation in a fermenting bacterium is discussed. From a fermenting bacterium,Thermotoga maritima, Kuhns, Trifunovi c et al. purify a complex that includes a respiratory enzyme, Rnf. They find that the Rnf complex requires Na(+)for activity and that it catalyzes Na(+)transport in liposomes. This study shows that the Rnf complex is indeed an ion translocating, respiratory enzyme