ALANINE, NOT AMMONIA, IS EXCRETED FROM N-2-FIXING SOYBEAN NODULE BACTEROIDS

Symbiotic nitrogen fixation, the process whereby nitrogen-fixing bacte ria enter into associations with plants, provides the major source of nitrogen for the biosphere, Nitrogenase, a bacterial enzyme, catalyzes the reduction of atmospheric dinitrogen to ammonium, In rhizobialegum inous plant symbioses, the current model of nitrogen transfer from the symbiotic form of the bacteria, call ed a bacteroid, to the plant is that nitrogenase-generated ammonia diffuses across the bacteroid membr ane and is assimilated into amino acids outside of the bacteroid, We p urified soybean nodule bacteroids by a procedure that removed contamin ating plant proteins and found that alanine was the major nitrogen-con taining compound excreted. Bacteroids incubated in the presence of N-1 5(2) excreted alanine highly enriched in N-15. Th, ammonium in these a ssays neither accumulated significantly nor was enriched in N-15, Th, results demonstrate that a transport mechanism rather than diffusion f unctions at this critical step of nitrogen transfer from the bacteroid s to the plant host. Alanine may serve only as a transport species, bu t this would permit physiological separation of the transport of fixed nitrogen from other nitrogen metabolic functions commonly mediated th rough glutamate.