Movements of Ammonia in Rhizobium leguminosarum

Histidine-grown cells of Rhizobium leguminosarum can accumulate more than 100 mm free ammonia in the medium. Growth and respiration are unaffected by these concentrations of ammonia. Measurements of internal and external ammonia concentrations for cells grown on histidine or NH4Cl as nitrogen sources suggest rapid equilibration of ammonia across the cell membrane under conditions where actively accumulated solutes are retained. Implications of these results for movement of ammonia out of N2-fixing bacteroids are discussed.