The cyclic ␤-(133),␤-(136)-D-glucan synthesis locus of Bradyrhizobium japonicum is composed of at least two genes, ndvB and ndvC. Mutation in either gene affects glucan synthesis, as well as the ability of the bacterium to establish a successful symbiotic interaction with the legume host soybean (Glycine max). B. japonicum strain AB-14 (ndvB::Tn5) does not synthesize ␤-glucans, and strain AB-1 (ndvC::Tn5) synthesizes a cyclic ␤-glucan lacking ␤- (136)

Acetylene reduction has been demonstrated in pure cultures of rhizobia. The requirements and conditions necessary for the activity in Rhizobium sp. 32H1 are described. The most important factors are a low cell density and a very low oxygen concentration.

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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.