Photoproduction of ammonium ion from N2 inRhodospirillum rubrum

Abstract

NH ₄ ⁺ excretion was undetectable in N₂-fixing cultures ofRhodospirillum rubrum (S-1) and nitrogenase activity in these cultures was repressed by the addition of 10 mM NH ₄ ⁺ to the medium. The glutamate analog,l-methionine-dl-sulfoximine (MSX), derepressed N₂ fixation even in the presence of 10 mM extracellular NH ₄ ⁺ . When 10 mg MSX/ml was added to cultures just prior to nitrogenase induction they developed nitrogenase activity (20% of the control activities) and excreted most of their fixed N₂ as NH ₄ ⁺ . Nitrogenase activities and NH ₄ ⁺ production from fixed N₂ were increased considerably when a combined nitrogen source, NH ₄ ⁺ (>40 μmoles NH ₄ ⁺ /mg cell protein in 6 days) orl-glutamate (>60 μmoles NH ₄ ⁺ /mg cell protein in 6 days) was added to the cultures together with MSX. Biochemical analysis revealed thatR. rubrum produced glutamine synthetase and glutamate synthase (NADP-dependent) but no detectable NADP-dependent glutamate dehydrogenase. The specific activity of glutamine synthetase was observed to be maximal when nitrogenase activity was also maximal. Nitrogenase and glutamine synthetase activities were repressed by NH ₄ ⁺ as well as by glutamate. The results demonstrate that utilization of solar energy to photoproduce large quantities of NH ₄ ⁺ from N₂ is possible with photosynthetic bacteria by interfering with their regulatory control of N₂ fixation.