Two adjacent genes in Bradyrhizobium japonicum, fbcF and fbcH, encode the Rieske iron sulfur protein and cytochromes b and c1, characteristic constituents of the respiratory complex III. Remarkably, fbcH is a single gene of which the 5' half codes for cytochrome b and the 3' half codes for cytochrome c1. Experimental evidence suggests that a large FbcH precursor is posttranslationally processed into the two proteins. B. japonicum fbcF and fbcH insertion mutants grow aerobically but are unable to fix nitrogen in root nodule symbiosis with soybean. Thus, fbcF and fbcH are symbiotically essential. We propose that B. japonicum makes use of a cytochrome bc1-containing respiratory chain on its way to become a microaerobic endosymbiont, whereas under aerobiosis, respiration can occur by a bc1-independent pathway.