Summary: Nitrogenase from the strain ORS571, which forms both root and stem nodules on the tropical plant , was purified from free-living diazotrophically grown organisms. The enzyme complex was resolved into two protein components resembling those obtained from other diazotrophs. Both components were purified to homogeneity as judged by SDS-gel electrophoresis. Component 1, a Mo-Fe protein, had a of 219000 and contained 1·2 g-atoms Mo mol, 22·5 g-atoms Fe moland 20·5 g-atoms acid-labile S mol. It consisted of two types of subunit of 56000 and 59000. The specific activity [nmol product formed min(mg protein)] of component 1, when assayed in the presence of an optimum concentration of component 2 (molar ratio 1:40), was 1250 for acetylene reduction and 1090 for hydrogen evolution. Component 2, an Fe protein, had a of 74000 and contained 3·1 g-atoms Fe moland 3·1 g-atoms acid-labile S mol. It consisted of a single type of subunit of 36000. The specific activity of component 2, when assayed in the presence of an optimum concentration of component 1 (molar ratio 1:1) was 1700 for acetylene reduction and 1305 for hydrogen evolution. Nitrogenase activity of strain ORS571 was subject to ‘switch-off’ when ammonia was added to a N-fixing culture. This effect was independent of protein synthesis and was reversible. Nitrogenase components 1 and 2 isolated from ‘switched-off’ organisms were purified to homogeneity. Both components had the same mobility on SDS-gels as those isolated from active cultures. ‘Switch-off’ resulted in a decrease in the specific activity of component 2 from 1700 to 280. Component 1 remained fully active. The addition of Mnto a crude extract containing inactivated nitrogenase did not restore activity.


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