Table 1. A. vinelandii Genes Involved in Mo-Dependent N2 Fixation and Roles of Their Products.
| genea | Nif phenotypeb | identity/role(s) |
|---|---|---|
| nifH | – | Fe protein. Obligate electron donor to the MoFe protein. Also required for FeMo-co biosynthesis and apo-MoFe maturation. |
| nifD | – | MoFe protein α-subunit. FeMo-co binding subunit. Forms an α2β2 tetramer with NifK. |
| nifK | – | MoFe protein β-subunit. Forms an α2β2 tetramer with NifD. |
| nifT | + | Unknown |
| nifY | + | Interacts with apo-MoFe protein prior to FeMo-co insertion. |
| nafA | + | Unknown function. |
| nifE | – | Scaffold for NifB-co maturation into FeMo-co. Forms an α2β2 tetramer with NifN. |
| nifN | – | Scaffold for NifB-co maturation into FeMo-co. Forms an α2β2 tetramer with NifE. |
| nifX | + | Binds NifB-co and VK-cluster. Interacts with apo-NifEN. Proposed carrier of FeMo-co precursors. |
| nafC | + | Unknown function. |
| feSII | + | Shethna protein II. Nitrogenase protection against O2. |
| nifIscA | + | Fe–S scaffolding Nif protein. Contains a [4Fe-4S] cluster. |
| nifU | – | Supplies [Fe–S] clusters precursors for nitrogenase proteins and cofactors. |
| nifS | ± | Mobilizes S for [Fe–S] cluster synthesis on NifU. |
| nifV | ± | Homocitrate synthase. Involved in FeMo-co synthesis. |
| cysE1 | + | Serine O-acetyltransferase. Participates in cysteine formation. |
| nafH | + | Binds to apo-MoFe protein preceding P-cluster maturation. |
| nifW | ± | Associates with apo-MoFe protein preceding P-cluster maturation. |
| nifZ | ± | Involved in P-cluster maturation in combination with the Fe protein. |
| nifM | – | Essential for the maturation of the Fe protein. Proposed peptidyl-prolyl cis–trans isomerases that would act over the Pro258 residue of NifH. |
| clpx2 | + | Regulates NifEN and NifB protein levels. |
| nifF | + | Flavodoxin. Physiological electron donor to the Fe protein. |
| nifL | + | Sensor protein. Acts as NifA antiactivator. |
| nifA | – | Transcriptional activator. Forms a two-component regulatory system with NifL. |
| nifB | – | Essential for FeMo-co synthesis. Catalyzes NifB-co formation, the first committed step in FeMo-co biosynthesis. |
| fdxN | ± | 2x[4Fe-4S] ferredoxin. Required for NifB in the formation of NifB-co. |
| nifO | + | Required for simultaneous activation of nitrogenase and nitrate reductase. |
| nifQ | – | Processes Mo prior to its incorporation into FeMo-co. |
| nafW | + | Rhodanase-like protein. Also called rhdN. Unknown function. |
| nafZ | + | Glutaredoxin-like protein. Also called grx5nif. Unknown function. |
| rnfAc | ± | Transmembrane protein. Part of the Rnf complex, which is involved in supplying low potential electrons to the nitrogenase. |
| rnfBc | ± | Contains two [Fe–S] cluster binding sites. It is part of the Rnf complex. |
| rnfCc | ± | Contains [4Fe-4S] binding site. Electron transport component of the Rnf complex. |
| rnfDc | ± | Transmembrane protein of the Rnf complex. Includes a flavin binding motive. |
| rnfGc | ± | Last electron acceptor component of the Rnf complex. Contains FMN binding site. |
| rnfEc | ± | Transmembrane protein of the Rnf complex. |
| rnfHc | ± | Soluble protein of the rnf gene cluster. |
| nafY | + | In A. vinelandii, γ subunit of apo-MoFe protein. Stabilizes apo-MoFe protein prior to FeMo-co insertion. Also binds FeMo-co specifically. |
| mosA | + | Mo storage protein α subunit. Forms an α3β3 hexamer with MosB. |
| mosB | + | Mo storage protein β subunit. |
a
Minimum set of essential Mo-nitrogenase genes is shown in bold.
b
+ Diazotrophic growth, ± slow diazotrophic growth, – no diazotrophic growth.
c
Phenotype of rnf genes was determined by deleting the entire rnf operon.