Strategies for increased ammonium production in free-living or plant associated nitrogen fixing bacteria

Strategies considered and studied for achieving ammonium excretion in nitrogen fixing bacteria include 1) inhibition of ammonium assimilation and 2) interference with the mechanisms by which ammonium inhibits either nitrogenase synthesis or activity. These aspects of nitrogen fixation have been best studied in diazotrophic Proteobacteria and Cyanobacteria and those of the former are reviewed in this paper. Ammonium assimilation by glutamine synthetase (GS) can be diminished or prevented by treatment of bacteria with chemicals that inhibit GS activity and in some diazotrophs, such treatment results in excretion of up to 15mM ammonium into liquid growth medium. Also, mutants with altered GS activity, isolated by selection for resistance to GS inhibitors, often excrete ammonium. In Proteobacteria, ammonium inhibits nitrogenase activity and/or synthesis, the latter by preventing activity or expression of NifA, a transcriptional activator required for expression of other nif genes. In Azotobacter vinelandii, ammonium inhibits NifA activity but not its synthesis; NifL mediates this effect by interacting directly with NifA causing its inactivation. In nifL insertion mutants, NifA is constitutively active and up to 10 mM ammonium is excreted during nitrogen fixation. GlnD insertion/deletion mutations are unable to be stably maintained in A. vinelandii wild type but are stable and viable in a mutant that produces constitutively active GS (cannot be adenylylated). This confirms the hypothesis that GlnD is required for activity of GS, an essential enzyme in A. vinelandii. In addition, the stable glnD mutants are Nif, supporting also the previous conclusion that GlnD is involved in mediating NifL/NifA interaction. Mechanisms of inhibition of synthesis or activity of NifA by ammonium in other diazotrophs are discussed and compared.