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A purification procedure is described for the components of Bacillus polymyxa nitrogenase. The procedure requires the removal of interfering mucopolysaccharides before the two nitrogenase proteins can be purified by the methods used with other nitrogenase components. The highest specific activities obtained were 2750 nmol C2H4 formed . min-1 . mg-1 MoFe protein and 2521 nmol C2H4 formed . min-1 . mg-1 Fe protein. The MoFe protein has a molecular weight of 215 000 and contains 2 molybdenum atoms, 33 iron atoms and 21 atoms of acid-labile sulfur per protein molecule. The Fe protein contains 3.2 iron atoms and 3.6 acid-labile sulfur atoms per molecule of 55 500 molecular weight. Each Fe protein binds two ATP molecules. The EPR spectra are similar to those of other nitrogenase proteins. MgATP changes the EPR of the Fe protein from a rhombic to an axial-type signal.

The catalytically inactive complex generated by the combination of the Azotobacter vinelandii MoFe protein (Av1) and the Clostridium pasteurianum Fe protein (Cp2) inhibits N2 reduction, C2H3 reduction, H+ reduction and ATP hydrolysis catalyzed by the homologous nitrogenases. Kinetic data indicate that the inactive complex consists of two molecules of Cp2 to one molecule of Av1, with values for the inhibitor constant in the range of 1--10 nM. Inhibition of C. pasteurianum nitrogenase by Av1 produces a lag phase in acetylene reduction that increases with increasing Av1. The lag phase is found only at levels of Av1 sufficient to keep the ratio of Cp2 : Cp1 lower than 2. Gel filtration of a mixture of Av1 and Cp2 provides evidence for complex formation and indicates that each Av1 molecule binds more than one Cp2 molecule. The Av1-Cp2 complex binds two molecules of MgATP per molecule of Cp2. MgATP is not required for complex formation, but complex formation lowers the MgATP-Cp2 dissociation constant approx. 3-fold. Av1 protects the iron-sulfur center in Cp2 completely against the MgATP-induced reaction with chelators. This provides additional evidence for formation of the Av1-Cp2 complex and together with the results of the MgATP-binding studies suggests that the two binding sites for MgATP are some distance away from the iron-sulfur site on Cp2.