Nitrogen Fixation and Carbon Dioxide Assimilation in Rhizobium japonicum

In free-living Rhizobium japonicum cultures, the stimulatory effect of CO(2) on nitrogenase (acetylene reduction) activity was mediated through ribulose bisphosphate carboxylase activity. Two mutant strains (CJ5 and CJ6) of R. japonicum defective in CO(2) fixation were isolated by mitomycin C treatment. No ribulose bisphosphate carboxylase activity could be detected in strain CJ6, but a low level of enzyme activity was present in strain CJ5. Mutant strain CJ5 also exhibited pleiotropic effects on carbon metabolism. The mutant strains possessed reduced levels of hydrogen uptake, formate dehydrogenase, and phosphoribulokinase activities, which indicated a regulatory relationship between these enzymes. The CO(2)-dependent stimulation of nitrogenase activity was not observed in the mutant strains. Both mutant strains nodulated soybean plants and fixed nitrogen at rates comparable to that of the wild-type strain.