CELLULAR ATP LEVELS AND NITROGENASE SWITCHOFF UPON OXYGEN STRESS IN CHEMOSTAT CULTURES OF AZOTOBACTER-VINELANDII

When Azotobacter vinelandii, growing diazotrophically in chemostat cul ture, was subjected to sudden increases in the ambient oxygen concentr ation (oxygen stress), nitrogenase activity was switched off and cellu lar ATP pools decreased at rates depending on the stress level, Follow ing a fast decrease, the ATP pool approached a lower level. When the s tress was released, these effects were reversed. The reversible decrea se of the ATP pool upon oxygen stress could also be observed with cult ures assimilating ammonium and, at the same time, fixing dinitrogen be cause of growth at a high C/N ratio but not with cultures growing only at the expense of ammonium. When strains OP and UW136 of A. vinelandi i were subjected to long-term increases in ambient oxygen, the sizes o f cellular ATP pools eventually started to increase to the level befor e stress and diazotrophic growth resumed. The cytochrome d-deficient m utant MK5 of A. vinelandii, however, impaired in aerotolerant diazotro phic growth; was unable to recover from stress on the basis of its ATP pool. The results suggest that adaptation to higher ambient oxygen de pends on increased ATP synthesis requiring increased electron how thro ugh the entire respiratory chain, which is possible only in combinatio n with the more active, yet possibly uncoupled, branch terminated by c ytochrome d. It is proposed that the decrease of the cellular ATE leve l under oxygen; stress-resulted from the increased energy and electron donor requirement of nitrogenase in reacting with oxygen.