Mechanistic studies of O2-resistant N2-fixation in N2-fixing Escherichia coli

Escherichia coli carrying the entire nif gene cluster from Klebsiella pneumoniae on a multicopy plasmid becomes more O2-resistant in a N-free medium as a result of the integration of the nif gene cluster into the chromosome and the loss of the plasmid (H.Iwahashi and J.Someya, Biochem. Biophys. Res. Comm. 1990, 168: 288–294). Our purpose is to characterize the physiological reason why the strain became O2-resistant by measuring the levels of nif proteins in cells under microaerobic conditions. The O2-resistant strain had a higher amount of NifH and a lower amount of NifL under microaerobic conditions (compared to that under anaerobic conditions), while the parent strain showed the opposite characteristics. Thus, the biochemical mechanism of the O2-resistant strain is attributed to the strain's ability to synthesize and maintain a high amount of NifH and a low amount of NifL under microaerobic conditions. © Rapid Science Ltd. 1998