The Genetic Analysis of Nitrogen Fixation, Oxygen Tolerance and Hydrogen Uptake in Azotobacters [and Discussion]

Azotobacters are important in nitrogen-fixation research because of their ability to synthesize at least two alternative forms of nitrogenase and also because of their high tolerance to oxygen. Approaches to studying genes in azotobacters involved in these and related processes include the analysis of mutants, hybridization to genes of other organisms, and also complementation of K. pneumoniae and E. coli mutants by azotobacter DNA. Eight to ten different regions of the genome may contain DNA involved in nitrogen fixation in A. chroococcum . The largest of these is about 25 kilobases (kb) in length and resembles the nif cluster of K. pneumoniae to some extent. Other regions include those hybridizing to fixABC genes of rhizobia and those thought to be involved in the Va-based alternative nitrogenase. Regulation of expression of genes for Mo nitrogenase in A. vinelandii involves, as in K. pneumoniae , ntrA and nifA genes, but unlike K. pneumoniae , not ntrC . Another regulatory gene, called nfrX , has also been identified. Mutants of A. chroococcum with increased sensitivity to oxygen (Fos - ) have been isolated and their phenotypes related to mechanisms of oxygen tolerance. Two are characterized as being deficient in citrate synthase and PEP carboxylase, respectively; these indicate that efficient operation of the TCA cycle is important for respiratory protection of nitrogenase. Finally, genetic studies of hydrogen uptake in A. chroococcum include the characterization of 15 kb of hup DNA by hybridization and mutant-complementation experiments.

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