Inhibition of iron-molybdenum cofactor biosynthesis by L127 Delta NifH andevidence for a complex formation between L127 Delta NifH and NifNE

Besides serving as the obligate electron donor to dinitrogenase during nitr ogenase turnover, dinitrogenase reductase (NifH) is required for the biosyn thesis of the iron-molybdenum cofactor (FeMo-co) and for the maturation of alpha(2)beta(2) apo-dinitrogenase (apo-dinitrogenase maturation). In an att empt to understand the role of NifH in FeMo-co biosynthesis, a site-specifi c altered form of NifH in which leucine at position 127 has been deleted, L 127 Delta, was employed in in vitro FeMo-co synthesis assays, This altered form of NifH has been shown to inhibit substrate reduction by the wild-type nitrogenase complex, forming a tight protein complex with dinitrogenase. T he L127 Delta NifH was found to inhibit in vitro FeMo-co synthesis by wild- type NifH as detected by the gamma gel shift assay, Increasing the concentr ation of NifNE and NifB-cofactor (NifB-co) relieved the inhibition of FeMo- co synthesis by L127 Delta NifH. The formation of a complex of L127 Delta N ifH with NifNE was investigated by gel filtration chromatography, We herein report the formation of a complex between L127 Delta NifH and NifNE in the presence of NifB-co, This work presents evidence for one of the possible r oles for NifH in FeMo-co biosynthesis, Le. the interaction of NifH with a N ifNE.NifB-co complex.