<p>This entry represents several Nif (B, X and Y) proteins, which are involved in the biosynthesis of the iron-molybdenum cofactor (FeMo-co) found in the dinitrogenase enzyme of the nitrogenase complex in nitrogen-fixing bacteria. The nitrogenase complex catalyses the reduction of atmospheric dinitrogen to ammonia, and is composed of an iron metalloprotein (dinitrogenase reductase; homodimer of NifH; <db_xref db="INTERPRO" dbkey="IPR000392"/>) and a Fe-Mo metalloprotein (dinitrogenase; heterotetramer of NifD and NifK; <db_xref db="INTERPRO" dbkey="IPR000318"/>). The pathway for the synthesis of the Fe-Mo cofactor involves several proteins, including NifB, NifE, NifH, NifN, NifQ, NifV and NifX. NifB appears to be an iron-sulphur source for FeMo-co biosynthesis, while NifX may be associated with the mature FeMo-co, in particular with the addition of homocitrate during the last step of biosynthesis [<cite idref="PUB00016259"/>]. The NifX protein shows sequence similarity with the C terminus of NifB [<cite idref="PUB00016260"/>], as well as to the conserved protein MTH1175 from the archaeon <taxon tax_id="187420">Methanobacterium thermoautotrophicum</taxon>, which displays a ribonuclease H-like motif of three layers, alpha/beta/alpha, with a single mixed beta-sheet [<cite idref="PUB00015608"/>].</p> Dinitrogenase iron-molybdenum cofactor biosynthesis