Effect of Oxygen on the Synthesis, Activity and Breakdown of the Rhizobium Denitrification System

SUMMARY: The synthesis, activity and breakdown of the denitrifying enzymes of Rhizobium japonicum, R. lupini and R. meliloti were found to be regulated by O2. Nitrogen oxide reductases were present in anaerobically grown and symbiotic R. japonicum, but in the case of organisms that had been grown aerobically the enzymes were induced only after a period of incubation under anaerobic conditions. Activity of the denitrification system that had been induced in aerobically grown cells was inhibited by O2. Denitrification by anaerobically grown cells and bacteroids was stimulated by 5% O2. Air inhibited denitrification completely. Little loss of denitrifying activity was shown by cells incubated in 5% O2, but cells incubated at 10% O2 showed a rapid loss of denitrification activity.