MECHANISTIC STUDIES OF O-2-RESISTANT N-2-FIXATION IN N-2-FIXING ESCHERICHIA-COLI

Escherichia coli carrying the entire nif gene cluster from Klebsiella pneumoniae on a multicopy plasmid becomes more O-2-resistant in a N-fr ee medium as a result of the integration of the nif gene cluster into the chromosome and the loss of the plasmid (H.lwahashi and J.Someya, B iochem. Biophys. Res. Comm. 1990, 168: 288-294). Our purpose is to cha racterize the physiological reason why the strain became O-2-resistant by measuring the levels of nif proteins in cells under microaerobic c onditions. The O-2-resistant strain had a higher amount of NifH and a lower amount of NifL under microaerobic conditions (compared to that u nder anaerobic conditions), while the parent strain showed the opposit e characteristics. Thus, the biochemical mechanism of the O-2-resistan t strain is attributed to the strain's ability to synthesize and maint ain a high amount of NifH and a low amount of NifL under microaerobic conditions.