INCORPORATION OF IRON AND SULFUR FROM NIFB COFACTOR INTO THE IRON-MOLYBDENUM COFACTOR OF DINITROGENASE

NifB-co is an iron- and sulfur-containing precursor to the iron-molybd enum cofactor (FeMo-co) of dinitrogenase. The synthesis of NifB-co req uires at least the product of the nifB gene. Incorporation of Fe-55 an d S-35 from NifB-co into FeMo-co was observed only when all components of the in vitro FeMo-co synthesis system were present. Incorporation of iron and sulfur from NifB-co into dinitrogenase was not observed in control experiments in which the apodinitrogenase (lacking FeMo-co) w as initially activated with purified, unlabeled FeMo-co or in assays w here NifB-co was oxygen-inactivated prior to addition to the synthesis system. These data clearly demonstrate that iron and sulfur from acti ve NifB-co are specifically incorporated into FeMo-co of dinitrogenase and provide direct biochemical identification of an iron-sulfur precu rsor of FeMo-co. Under different in vitro FeMo-co synthesis conditions , iron and sulfur from NifB-co were associated with at least two other proteins (NIFNE and gamma) that are involved in the formation of acti ve dinitrogenase. The results presented here suggest that multiple FeM o-co processing steps might occur on NIFNE and that FeMo-co synthesis is most likely completed prior to the association of FeMo-co with gamm a.