<p>The nitrogen fixation protein NifU is required for full activation of the metalloenzyme nitrogenase, the catalytic component of biological nitrogen fixation. In <taxon tax_id="354">Azotobacter vinelandii</taxon>, it is a homodimer; each subunit contains one 2Fe-2S cluster [<cite idref="PUB00016779"/>], which is proposed to have a redox function involving the release of Fe for the biosynthesis of the Fe-S cluster at the labile rubredoxin-like site [<cite idref="PUB00016841"/>].</p>