Incorporation of molybdenum into the iron-molybdenum cofactor of nitrogenase

The biosynthesis of the iron-molybdenum cofactor (FeMo-co) of dinitrogenase was investigated using Mo-99 to follow the incorporation of Mo into precur sors. Mo-99 label accumulates on dinitrogenase only when all known componen ts of the FeMo-co synthesis system, NfH, NifNE, NifB-cofactor, homocitrate, MgATP, and reductant, are present. Furthermore, Mo-99 label accumulates on ly on the gamma protein, which has been shown to serve as a chaperone/inser tase for the maturation of apodinitrogenase when all known components are p resent, It appears that only completed FeMo-co can accumulate on the gamma protein. Very little FeMo-co synthesis was observed when all known componen ts are used in purified forms, indicating that additional factors are requi red for optimal FeMo-co synthesis. Mo-99 did not accumulate on NifNE under any conditions tested, suggesting that Mo enters the pathway at some other step, although it remains possible that a MO-containing precursor of FeMo-c o that is not sufficiently stable to persist during gel electrophoresis occ urs but is not observed. Mo-99 accumulates on several unidentified species, which may be the additional components required for FeMo-co synthesis. The molybdenum storage protein was observed and the accumulation of Mo-99 on t his protein required nucleotide.