The quaternary structure of nitrogenase recombined complex (Mo-Fe protein + Fe protein) from Azotobacter vinelandii has been studied by electron microscopy for comparison with the structure of the Mo-Fe protein proposed earlier. Computer averaging of the images showed that one of the projections of the Mo-Fe protein could be characterized by twofold rotational symmetry and the molecule itself has 222 pseudosymmetry. Nitrogenase recombined complex in the same projection revealed additional protein densities that are associated with the Fe protein. It has been suggested that the Fe protein dimer in nitrogenase complex is located in cavities of the central part of the Mo-Fe protein.
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