The respiratory activities and cytochrome contents of nitrogen-fixing continuous cultures of (NCIB 8003) increased with the partial pressure of oxygen encountered during growth. Above 0.6 atm., wash-out of the culture occurred. Acetylene reduction by culture samples of low respiratory activity was far more easily inhibited by oxygenation than was that of samples of high respiratory activity, though their maximum acetylene-reducing activities at their optimal O values were similar. Inhibition by oxygen was reversible after mild oxygenation: 70 to 100% of the original activity returned immediately when the degree of oxygenation was decreased. Irreversible inhibition occurred after vigorous oxygenation and was associated with a loss of activity in cell-free extracts, which was restored by adding the oxygen-sensitive protein component of Azotobacter nitrogenase. These observations support earlier proposals that augmented respiration can scavenge oxygen from the nitrogen-fixing site and that a conformational change in the state of nitrogenase can prevent damage to the enzyme by oxygen. Vigorous aeration, however, may overcome these protective mechanisms.


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