Evidence for an ammonium transport system in the N2-fixing phototrophic bacteriumRhodospirillum rubrum

An ammonium transport system in the phototrophic N₂-fixing bacteriumRhodospirillum rubrum was characterized by using the uptake of¹⁴C-methylamine as a probe.Uptake showed saturation kinetics with an apparentK _m =110 M. It was competitively inhibited by ammonium (K _i =7 M). Uptake exhibited a narrow pH maximum around pH 7.0.Up to 200-fold gradients across the membrane were formed within 40–60 min. Gradient formation was inhibited by carbon starvation, azide or cyanide. Pre-accumulated methylamine was released by ammonium pulses to more than 80%, indicating only minor metabolization.The synthesis of the transport system was repressed by ammonium in high concentrations.