@article{mbs:/content/journal/micro/10.1099/00221287-131-6-1449, author = "RAMOS, JUAN L. and ROBSON*, ROBERT L.", title = "Isolation and Properties of Mutants of Azotobacter chroococcum Defective in Aerobic Nitrogen Fixation", journal= "Microbiology", year = "1985", volume = "131", number = "6", pages = "1449-1458", doi = "https://doi.org/10.1099/00221287-131-6-1449", url = "https://www.microbiologyresearch.org/content/journal/micro/10.1099/00221287-131-6-1449", publisher = "Microbiology Society", issn = "1465-2080", type = "Journal Article", abstract = "Mutants of Azotobacter chroococcum which showed unusually O2-sensitive N2 fixation when grown on sugars were isolated following mutagenesis with nitrosoguanidine or the transposon Tm1. These mutants we called Fos– (inability to fix N2 on sugars). Aerotolerant growth of Fos– mutants was, in general, restored by carboxylic acids, Ca2+ or a combined-N source. Two main groups of mutants were distinguished: (1) regulatory mutants which failed to synthesize nitrogenase under aerobic conditions and which were complemented by the Klebsiella pneumoniae nifA gene product; (2) mutants affected at different steps in intermediary metabolism. The latter strains exhibited low respiration rates or high apparent K s(O2) values (when grown on sucrose) compared with the parent strain. They seemed to be defective in respiratory protection of nitrogenase. The restoration of aerotolerant N2-dependent growth of Fos– mutants by carboxylic acids was correlated with their ability to induce a decrease in the apparent K s(O2) value; however, in both the mutant and the parent strains maximum respiration rates did not change significantly. The Ca2+ requirement for diazotrophic growth in A. chroococcum seems to be related to the capability of these organisms to fix N2 in air. Ca2+ 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 acid cycle and/or at the level of tricarboxylic acid cycle enzymes per se. It is suggested that respiratory protection requires the maintenance of high respiratory rates even at low O2 tension.", }