BRADYRHIZOBIUM-JAPONICUM CYTOCHROME C(550) IS REQUIRED FOR NITRATE RESPIRATION BUT NOT FOR SYMBIOTIC NITROGEN-FIXATION

Bradyrhizobium japonicum possesses three soluble c-type cytochromes, c(550), c(552), and c(555). The genes for cytochromes c(552) (cycB) and c(555) (cycC) were characterized previously, Here we report the cloning, sequencing, and mutational analysis of the cytochrome c(550) gene (cycA), A B, japonicum mutant with an insertion in cycA failed to synthesize a 12-kDa c-type cytochrome. This protein was detectable in the cycA, mutant complemented with cloned cycA, which proves that it is the cycA gene product. The cycA mutant, a cycB-cycC double mutant, and a cycA-cycB-cycC triple mutant elicited N-2-fixing root nodules on soybean (Nod(+) Fix(+) phenotype); hence, none of these three cytochromes c is essential for respiration supporting symbiotic N-2 fixation. However, cytochrome c(550), in contrast to cytochromes c(552) and c(555), was shown to be essential for anaerobic growth of B. japonicum, using nitrate as the terminal electron acceptor.