Reversible and irreversible inactivation of cellular nitrogenase upon oxygen stress in Azotobacter vinelandii growing in oxygen controlled continuous culture.

Azotobacter vinelandii growing in oxygen controlled chemostat culture was subjected to sudden increases of ambient oxygen concentrations (oxygen stress) after adaptation to different oxygen concentrations adjustable with air (100% air saturation corresponds to 225 +/- 14 microM O2). Inactivations of cellular nitrogenase during stress (switch off) as well as after release of stress (switch on) were evaluated in vivo as depending on stress duration and stress height (delta pO2). Switch off was at its final extent within 1 min of stress. The extent of switch off, however, increased with stress height and was complete at delta pO2 between 8-10% air saturation irrespective of different oxygen concentrations the organisms were adapted to before stress, indicating that switch off is adaptable. Inactivation of nitrogenase measureable after switch on represents irreversible loss of activity. Irreversible inactivation was at its characteristic level within less than 3 min of stress and at a delta pO2 of less than 1% air saturation. The level of irreversible inactivation increased linearly with the oxygen concentration the organisms were adapted to before stress. Thus adaptation of cells to increased oxygen concentrations did not prevent increased susceptibility of nitrogenase to irreversible inhibition during oxygen stress. The fast response of irreversible inactivation at low stress heights suggests that it takes place already during stress. Thus switch off comprised both a reversible and an irreversible phase. The data showed that reversible inactivation of nitrogenase was less susceptible to oxygen stress than irreversible inactivation.(ABSTRACT TRUNCATED AT 250 WORDS)