Evidence for the lack of oxygen repression of Fe−Mo protein in mutants ofAnabaena variabilis

Abstract

Mutants ofAnabaena variabilis, unable to fix nitrogen under aerobic conditions, were used to determine whether nitrogenase synthesis is subject to O2 repression, as is the case in some heterotrophic bacteria. Nitrogenase activity in the mutants was induced as heterocysts matured under microaerophilic conditions. However, addition of 5% O2 to the assay system inhibited activity by 95%. Under aerobic conditions, nitrogenase activity in the mutants could not be detected, but an activity-independent, immunological assay showed that the Fe-Mo protein was present at levels similar to those found in wild-type and mutant strains induced microaerophilically. Reducing the O2 tension of an aerobically induced mutant resulted in a rate of nitrogenase activity induction twice the rate under continuous microaerophilic conditions. These results indicate that O2 does not repress Fe-Mo protein synthesis in these mutants.