title: Ammonia switch-off of nitrogen fixation in the methanogenic archaeon Methanococcus maripaludis: mechanistic features and requirement for the novel GlnB homologues, NifI(1) and NifI(2).
abstract: Ammonia switch-off is the immediate inactivation of nitrogen fixation that occurs when a superior nitrogen source is encountered. In certain bacteria switch-off occurs by reversible covalent ADP-ribosylation of the dinitrogenase reductase protein, NifH. Ammonia switch-off occurs in diazotrophic species of the methanogenic Archaea as well. We showed previously that in Methanococcus maripaludis switch-off requires at least one of two novel homologues of glnB, a family of genes whose products play a central role in nitrogen sensing and regulation in bacteria. The novel glnB homologues have recently been named nifI(1) and nifI(2). Here we use in-frame deletions and genetic complementation analysis in M. maripaludis to show that the nifI(1) and nifI(2) genes are both required for switch-off. We could not detect ADP-ribosylation or any other covalent modification of dinitrogenase reductase during switch-off, suggesting that the mechanism differs from the well-studied bacterial system. Furthermore, switch-off did not affect nif gene transcription, nifH mRNA stability, or NifH protein stability. Nitrogenase activity resumed within a short time after ammonia was removed from a switched-off culture, suggesting that whatever the mechanism, it is reversible. We demonstrate the physiological importance of switch-off by showing that it allows growth to accelerate substantially when a diazotrophic culture is switched to ammonia.
authors.affiliation: Department of Microbiology, University of Washington, Seattle, Washington 98195, USA.
authors.forename: P S
authors.initials: PS
authors.lastname: Kessler
authors.forename: C
authors.initials: C
authors.lastname: Daniel
authors.forename: J A
authors.initials: JA
authors.lastname: Leigh
mesh_terms: D000641:Ammonia / Q000378:metabolism*; D019843:Archaeal Proteins / Q000235:genetics*; D005798:Genes, Bacterial; D005812:Genes, Switch; D017017:Methanococcus / Q000235:genetics* / Q000378:metabolism; D009586:Nitrogen Fixation / Q000235:genetics; D010088:Oxidoreductases / Q000378:metabolism; D051922:PII Nitrogen Regulatory Proteins
chemical_list: D019843:Archaeal Proteins; C423625:NifI1 protein, Methanococcus maripaludis; C423626:NifI2 protein, Methanococcus maripaludis; D051922:PII Nitrogen Regulatory Proteins; D000641:Ammonia; D010088:Oxidoreductases; C029130:nitrogenase reductase