Five representative bacterial strains resembling , chosen from among forty strains isolated by three different procedures, were characterized by phenotypic and genotypic analyses. They were shown to belong to the species , or to a distinct, but closely related, cluster. In contrast to the initial isolates of this species, most of the new strains were able to fix N, as shown by their growth in nitrogen-free medium and by the acetylene reduction test. Moreover, DNA regions of these new strains were homologous to genes, confirming the presence of nitrogenase genes. The genes are probably chromosomally located, since no plasmid was detected in these strains. The presence of complete but defective genes in GA3 (the type strain, which does not fix N) was excluded, as no homology was found between genes and GA3 total DNA. The isolation conditions seem to determine the selection of N-fixing strains, since selection for H-chemolithoautotrophs gave no N-fixing isolates, whereas H-chemolithoheterotrophic selection conditions (with combined nitrogen) yielded a majority of N fixers.


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