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 cloni ng, sequencing, and mutational analysis of the cytochrome c(550) gene (cycA), A B, japonicum mutant with an insertion in cycA failed to synt hesize a 12-kDa c-type cytochrome. This protein was detectable in the cycA, mutant complemented with cloned cycA, which proves that it is th e 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 soyb ean (Nod(+) Fix(+) phenotype); hence, none of these three cytochromes c is essential for respiration supporting symbiotic N-2 fixation. Howe ver, 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.