TABLE 1.

nif gene products and their role (known or proposed) in nitrogen fixation

Gene>	Identity and/or role of gene product
nifH	Fe protein. Obligate electron donor to MoFe protein during nitrogenase turnover. NifH is also required for FeMo-co biosynthesis and apo-MoFe protein maturation.
nifD	α subunit of MoFe protein. Forms an α2β2 tetramer with the β subunit. The site of substrate reduction, FeMo-co, is within the α subunit of MoFe protein.
nifK	β subunit of MoFe protein. P-clusters are present at each αβ subunit-interface.
nifT	Unknown.
nifY/nafY	Chaperone for the apo-MoFe protein. NafY is also a FeMo-co carrier and is proposed to aid in the insertion of FeMo-co into apo-MoFe protein.
nifE	Forms α2β2 tetramer with NifN. Required for FeMo-co synthesis. Proposed to function as a scaffold on which FeMo-co is synthesized.
nifN	Required for FeMo-co synthesis. Tetramer with NifE.
nifX	Involved in FeMo-co synthesis. Accumulates an FeSMo-containing precursor.
nifU	Molecular scaffold for the formation of Fe-S cluster for nitrogenase components.
nifS	Involved in mobilization of S for Fe-S cluster synthesis and repair.
nifV	Homocitrate synthase, involved in FeMo-co synthesis.
nifW	Involved in stability of MoFe protein.
nifZ	Unknown.
nifM	Required for the maturation of NifH.
nifF	Flavodoxin. Physiological electron donor to NifH in K. pneumoniae.
nifL	Negative regulatory element.
nifA	Positive regulatory element.
nifB	Required for FeMo-co synthesis. Its metabolic product, NifB-co, is a specific Fe and S donor to FeMo-co.
fdxN	Ferredoxin. In R. capsulatus, it serves as electron donor to nitrogenase.
nifQ	Involved in FeMo-co synthesis. Proposed to function in early MoO42− processing.
nifJ	Pyruvate: flavodoxin (ferredoxin) oxidoreductase. Electron donor to Fe protein in K. pneumoniae.