Electrogenic ATPase activity on the peribacteroid membrane from soybean (Glycine max L. cv Bragg) root nodules is demonstrated. Membrane energization was monitored using suspensions of intact peribacteroid membrane-enclosed bacteroids (peribacteroid units; PBUs) and the fluorescent probe for membrane potential (A#), bis-(3-phenyl-5-oxoisoxazol-4yl) pentamethine ox-onol. Generation of a positive A4, across the peribacteroid membrane was dependent upon ATP, inhibited by N,N'-dicyclohexylcarbodiimide and vanadate, but insensitive to N-ethylmaleimide, azide, cyanide, oligomycin, and ouabain. The results suggest the presence of a single, plasma membrane-like, electrogenic ATPase on the peribacteroid membrane. The protonophore, carbonyl-cyanide m-chlorophenyl hydrazone, completely dissipated the established membrane potential. The extent of reduction in the steady state membrane potential upon addition of ions was used to estimate the relative permeability of the peribacteroid membrane to anions. By this criterion, the relative rates of anion transport across the peribacteroid membrane were: N03-> N02-> Cl-> acetate-> malate-. The observation that 10 millimolar N03-completely dissipated the membrane potential was of particular interest in view of the fact that N03-inhibits symbiotic nitrogen fixation. The possible function of the ATPase in symbiotic nitrogen fixation is discussed.Under nitrogen-limited conditions, the growth of legumes can be enhanced by an endosymbiotic association with nitrogen-fixing bacteria from the genera Bradyrhizobium and Rhizobium. Infection of the plant root by (Brady)rhizobia leads to the formation of root nodules which provide an environment suitable for symbiotic nitrogen fixation. Bacteroids [the symbiotic form of (Brady) rhizobia] within the infected cells of the nodule cortex are enclosed by the plant-derived PBM2.The PBM is formed initially via endocytosis of the plant cell plasma membrane (2,7,10,17,26), but subsequent proliferation appears to be supported by membrane flow from the biosynthetic endomembrane system (13,19 (27,28). Thus, it is to a large extent the PBM which prevents the symbiosis from degenerating into parasitism. The PBM, strategically positioned between macro-and microsymbiont, is likely to play a major role in the regulation of nitrogen fixation by controlling exchange of metabolites. The primary exchange which must occur across the PBM is the supply of a reduced-carbon compound(s) by the plant in return for reduced nitrogen (ammonia) from the bacteroid. Other exchanges, such as iron from the plant for haem from the bacteroid also appear essential (14). Additionally, some form of mineral or organic nitrogen must be supplied by the plant to the growing bacterial population prior to the onset of nitrogen fixation. We have recently begun a study of these exchanges using intact peribacteroid units (PBUs: PBM-enclosed bacteroids) isolated from soybean nodules and have described a dicarboxylate carrier on the PBM which catalyses transport of succinate and malate ...