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The nitrogenase enzyme of Klebsiella pneumoniae consists of two separable proteins, each with multiple subunits and one or more oxygen sensitive metallocenters. The wild-type nitrogenase proteins are stable to electrophoresis in high concentrations of urea under anaerobic conditions. Addition of Mg ϩ2 and ADP greatly increases the stability of the smaller Fe protein~from Ͻ4 to Ͼ6 M for full unfolding!, an effect directly analogous to stabilization in p21ras induced by Mg ϩ2 and GDP. Stabilization by Mg ϩ2 is slight for the holo MoFe protein~from ;1.5 to ;2.4 M! but more dramatic for the apo protein form of the MoFe protein accumulated by certain Fe protein~nifH gene! mutants. The potent product inhibitor of nitrogenase function, MgADP, increases stability of the MoFe protein more than Mg ϩ2 alone, to ;3.6 M, showing that nucleotides interact with the MoFe protein. Mutations of the nifM gene result in slower accumulation of less stable Fe protein, indicating that NifM is involved in correct folding of the Fe protein. Mutationally altered proteins are often difficult to purify for study because of their inherent instability, low expression level, or oxygen lability. Crude extracts of 11 different mutants of Fe protein~nifH gene! were examined by transverse urea gradient gels to rapidly screen for stabilizing interactions in the presence or absence of substrate or inhibitor analogs. Amino acid alterations D44N and R188C, at the interface of the dimer, in the vicinity of the nucleotide binding site~s!, have significantly lower stability than the wild-type enzyme in the absence of Mg ϩ2 but comparable stability in its presence, showing the importance of Mg ϩ2 in the subunit interactions. Mutations N163S and E266K, in which residues normally involved in hydrogen bonding far from the active site were altered, are more labile than the wild-type even with Mg ϩ2 added. Seven other mutants, though nonfunctional, did not appear altered in stability compared to the wild-type.