The nitrogenase proteins from eight organisms have been highly purified, and a survey of their cross-reactions shows that the nitrogenase proteins from a wide variety of organisms can interact with one another. An active cross-reaction is the complementary functioning of the MoFe protein and the Fe protein from different organisms. Of 64 possible combinations of component proteins, 8 yielded homologous nitrogenases (components from the same organism); 45 of the 56 possible heterologous crosses generated active hybrid nitrogenases; 4 heterologous crosses yielded no measurable nitrogenase activity but did form inactive tight-binding complexes; 6 crosses did not give measurable activity; and 1 cross was not made. All these crosses were assayed for acetylene reduction, and some also were assayed for ammonia formation, hydrogen evolution, and ATP hydrolysis activity. The activity generated by combining two complementary heterologous nitrogenase components depended on pH, component ratio, and protein concentration, the same factors that determine the activity of homologous nitrogenases. However, several crosses showed an unusual dependency on component ratio and protein concentration, and some cross-reactions showed interesting ATP hydrolysis activity.
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