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) and a Fe-Mo metalloprotein (dinitrogenase; heterotetramer of NifD and NifK). 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 . The NifX protein shows sequence similarity with the C-terminus of NifB , as well as to the conserved protein MTH1175 from the archaeon Methanobacterium thermoautotrophicum, which displays a ribonuclease H-like motif of three layers, alpha/beta/alpha, with a single mixed beta-sheet .