Oxygen resistant strain of N2-fixing Escherichia coli

Oxygen resistant N2-fixing Escherichia coli, which can grow at a high oxygen concentration in a nitrogen-free medium, were produced by several times of cultivation under a condition of 0.03% oxygen. Six isolated resistant strains could grow at an oxygen concentration ten times that of the parent strain. The nif-genes of these six strains were integrated into a chromosome. At low oxygen, they showed one third the nitrogenase activity to the parent strain, but more so at a high oxygen concentration. It is thus evident that the amount of nitrogenase protein in a cell is a factor determining the oxygen resistance of nitrogenase.