SUMMARY: An ammonium permease was derepressed when Rhizobium leguminosarum was grown in chemostat culture under conditions of nitrogen limitation. The ammonium permease was characterized by direct measurements with an ion-specific ammonium electrode. Cells grown under ammonia, nitrate, glutamate or methylamine limitation had permease activity, while those grown with excess (10 mM) ammonium chloride or glutamate did not. On transfer from N-limited to N-excess conditions, the permease disappeared rapidly, and all activity was lost within 18 h. Uptake by the permease was sensitive to azide, carbonyl cyanide m-chlorophenylhydrazone. 2,4-dinitrophenol, nigericin and valinomycin. The apparent for NH was 0-015 mM; ammonium uptake had a narrow maximum around pH 7.5. The internal ammonia concentration of N-limited cells was 0-4 mM, with up to 60-fold gradients of NH forming across the membrane within 30 min. Hydrazine and hydroxylamine strongly inhibited ammonium uptake, with methylamine, glutamine, aspartate and glycine less effective as inhibitors. Isolated pea bacteroids capable of transporting succinate did not possess the ammonium permease.


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