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 93 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 93 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 ammoniumperse is an important in vivo regulator ofnitrogen 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 the absence ofcombined nitrogen, heterocystous blue-green algae fix atmospheric N2 and produce NH3 which is subsequently assimilated by the GS2/GOGAT pathway (2,8,20,21). Exogenous NH3 inhibits nitrogenase activity and heterocyst differentiation in nitrogen fixing algae (3). Stewart and Rowell (20) have shown that this effect can be alleviated by the presence ofthe GS inhibitor L-methionine-DL-sulfoximine (MSX). This result prompted Stewart and Rowell (20) to suggest that the inhibition of nitrogen fixation was not caused by ammonium directly but rather by GS, GOGAT, or a product ofammonium assimilation. Subsequent work (14) suggested that the size of the glutamine pool was an important factor in nitrogenase regulation. Recent Cultures were maintained at 30°C in 400-ml culture bottles bubbled with filter sterilized laboratory air which had been passed through an activated charcoal filter in series with two gas scrubbers containing 1 N H2SO4 and distilled H20, respectively. This treatment removed particulate matter and gaseous ammonia while at the same time saturating the air with water vapor. Continuous illumination was provided by two Gro-lux fluorescent tubes. The quantum flux density was attenuated to 40 gE m-2 s-' with neutral density screening. Experiments were carried out using 20-ml portions of algal cultures in mid-logarithmic phase of growth (1-2 ,ug Chl a ml-'). These portions were incubated in 40-ml air-bubbled test tubes. Culture pH was adjusted to pH 7.1, 8.1, and 10.2 with phosphate (20 mM), bicine (20 mM), and CHES (20 mM), respectively.Nitrogenase Activity. Nitrogenase activity was determined by acetylene reduction. Acetylene reduction was measured using 2-ml aliquots placed in 1 5-ml test tubes sealed with rubber septa. The tubes were preincubated for 10 min in a shaking water bath and illuminated with a quantum...