Incorporation of the Iron and Sulfur from NifB-co into the Iron-Molibdenum Cofactor of Dinitrogenase

NifB-co, the product of the nifB gene, is involved in the biosynthesis of the ironmolibdenum cofactor (FeMo-co) of dinitrogenase. Incorporation of 55Fe and 35S from NifB-co into FeMo-co was investigated using the in vitro FeMo-co synthesis system. Fe and S from NifB-co were incorporated into dinitrogenase only under conditions where all components required for FeMo-co synthesis were present. Incorporation of Fe and S from NifB-co into dinitrogenase was not observed in control experiments in which apodinitrogenase was initially activated with purified, unlabelled FeMo-co or in assays where NifB-co was oxygen-inactivated prior to addition to the synthesis system. These data clearly demonstrate that Fe and S from active NifB-co are specifically incorporated into FeMo-co of dinitrogenase and provide the first biochemical identification of an FeS donor for FeMo-co synthesis.