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Ammonium suppresses nitrogenase activity in Anabaena flos-aquae (Lyng) Breb. at all pH values tested. l-Methionine-dl-sulfoximine at 1 millimolar totally inhibited glutamine synthetase, and 10 micromolar partially inhibited. Both concentrations protected nitrogenase activity from ammonium-induced suppression at pH 7.1 and 8.1. At pH 9.3 and 10.2, methionine sulfoximine did not alleviate the suppression of nitrogenase by ammonium. This pH-dependent protection of nitrogenase activity is a result of the noncompetitive inhibition of the ammonium transporter by methionine sulfoximine. At pH 7.1 and 8.2, ammonium is protonated and methionine sulfoximine inhibits its entry into the cell. At pH 9.3 and 10.2, unprotonated ammonia is abundant and may enter the cell independent of the transport system. The effects of ammonium are closely mimicked by the ammonium analog methylamine. These results suggest that ammonium per se is an important in vivo regulator of nitrogen fixation and its function can be mimicked by methylamine. Previous studies employing methionine sulfoximine may have to be re-evaluated in light of the inhibitory effects of methionine sulfoximine on the ammonium transporter.

In Vivo Nitrogenase Regulation by Ammonium and Methylamine and the Effect of MSX on Ammonium Transport in Anabaena flos-aquae