Immunological evidence for the capability of free-living Rhizobium japonicum to synthesize a portion of a nitrogenase component.

Immunodiffusion tests conducted under aerobic conditions demonstrated that cross-reactive material to antiserum prepared against the Mo-Fe protein component of nitrogenase from soybean nodule bacteroids was detectable in extracts of free-living Rhizobium japonicum cells cultured in a standard medium under: aerobic conditions; aerobic conditions with nitrate; aerobic conditions with ammonia; anaerobic conditions with nitrate; and anaerobic conditions with nitrate and ammonia. The most intense precipitin bands resulted from cross-reaction of the antiserum with extracts of cells cultured anaerobically with nitrate or anaerobically with ammonia and nitrate. Immunodiffusion experiments with crude bacteroid extract and purified Mo-Fe protein revealed a greater number of precipitin bands in tests conducted under aerobic conditions than those conducted under anaerobic conditions. These results indicate that some of the cross-reactive material observed under aerobic conditions resulted from breakdown of the Mo-Fe protein. Bacteroid extracts of nodules from plants supplied with ammonia exhibited only a trace of nitrogenase activity. The addition of an excess of the Fe protein component of nitrogenase, however, resulted in 270-fold enhancement of activity indication the presence of active Mo-Fe protein in these extracts. Our experiments together with results published elsewhere provide evidence that the genetic information for synthesis of a part of the Mo-Fe component of nitrogenase is carried by Rhizobium.