Mutants ofMutants of which showed unusually O-sensitive N fixation when grown on sugars were isolated following mutagenesis with nitrosoguanidine or the transposon Tml. These mutants we called Fos (inability to fix N on sugars). Aerotolerant growth of Fos mutants was, in general, restored by carboxylic acids, Ca or a combined-N source. Two main group of mutants were distinguished: (1) regulatory mutants which failed to synthesize nitrogenase under aerobic conditions and which were complemented by the gene product; (2) mutants affected at different steps in intermediary metabolism. The latter strains exhibited low respiration rates or high apparent (O) values (when grown on sucrose) compared with the parent strain. They seemed to be defective in respiratory protection of nitrogenase. The restoration of aerotolerant N-dependent growth of Fos mutants by carboxylic acids was correlated with their ability to induce a decrease in the apparent (O) value; however, in both the mutant and the parent strains maximum respiration rates did not change significantly. The Ca requirement for diazotrophic growth in A. chroococcum seems to be related to the capability of these organisms to fix N in air. Ca might be required for high fluxes in the tricarboxylic acid cycle and might act at the level of phosphoenolpyruvate carboxylase, which is involved in the replenishment of the tricarboxylic acad cycle and or at the level of tricarboxylic acid cycle enzymes It is suggested that respiratory protection requires the maintenance of high respiratory rates even at low O tension.


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