<p> The enzyme responsible for nitrogen fixation, thenitrogenase, shows a high degree of conservation of structure, function, and amino acid sequence across wide phylogenetic ranges. All known Mo-nitrogenases consist of two components, component I (also called dinitrogenase, or Fe-Mo protein), an alpha2beta2 tetramer encoded by the nifD and nifK genes, and component II (dinitrogenase reductase, or Fe protein) a homodimer encoded by the nifH gene [<cite idref="PUB00033167"/>, <cite idref="PUB00033168"/>]. Two operons, nifDK and nifEN, encode a tetrameric (alpha2beta2 and N2E2) enzymatic complex. Nitrogenase contains two unusual rare metal clusters; one of them is the iron molybdenum cofactor (FeMo-co), which is considered to be the site of dinitrogen reduction and whose biosynthesis requires the products of nifNE and ofsome other nif genes [<cite idref="PUB00033169"/>]. It has been proposed that NifNE might serve as a scaffold uponwhich FeMo-co is built and then inserted into component I [<cite idref="PUB00033170"/>].</p>