title: Nitrogen fixation: key genetic regulatory mechanisms.
abstract: The necessity to respond to the level of fixed nitrogen and external oxygen concentrations and to provide sufficient energy for nitrogen fixation imposes common regulatory principles amongst diazotrophs. The NifL-NifA system in Azotobacter vinelandii integrates the signals of redox, fixed-nitrogen and carbon status to regulate nif transcription. Multidomain signalling interactions between NifL and NifA are modulated by redox changes, ligand binding and interaction with the signal-transduction protein GlnK. Under adverse redox conditions (excess oxygen) or when fixed nitrogen is in excess, NifL forms a complex with NifA in which transcriptional activation is prevented. Oxidized NifL forms a binary complex with NifA to inhibit NifA activity. When fixed nitrogen is in excess, the non-covalently modified form of GlnK interacts with NifL to promote the formation of a GlnK-NifL-NifA ternary complex. When the cell re-encounters favourable conditions for nitrogen fixation, it is necessary to deactivate the signals to ensure that the NifL-NifA complex is dissociated so that NifA is free to activate transcription. This is achieved through interactions with 2-oxoglutarate, a key metabolic signal of the carbon status, which binds to the N-terminal GAF (cGMP-specific and stimulated phosphodiesterases, Anabaena adenylate cyclases and Escherichia coli FhlA) domain of NifA.
authors.affiliation: Department of Molecular Microbiology, John Innes Centre, Norwich NR4 7UH, UK.
authors.forename: I
authors.initials: I
authors.lastname: Martinez-Argudo
authors.forename: R
authors.initials: R
authors.lastname: Little
authors.forename: N
authors.initials: N
authors.lastname: Shearer
authors.forename: P
authors.initials: P
authors.lastname: Johnson
authors.forename: R
authors.initials: R
authors.lastname: Dixon
mesh_terms: D016948:Azotobacter vinelandii / Q000378:metabolism; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D005798:Genes, Bacterial; D009584:Nitrogen / Q000378:metabolism; D009586:Nitrogen Fixation / Q000235:genetics*; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; C061338:NifA protein, Bacteria; D014157:Transcription Factors; C084296:nifL protein, Bacteria; D009584:Nitrogen