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Molecular investigations on the dilemma of nitrogen‐fixing characteristics of Azotomonas

The ability of Azotomonas to reduce atmospheric nitrogen has been an issue of controversy since this genus was included in the family Azotobacteraceae. Here we report the investigations on the nitrogen fixation characteristics of Azotomonas fluorescens strain ATCC13544 and show that it can grow in both liquid medium and on solid agar plates made with modified BURK nitrogen‐free medium supplemented with ammonium acetate as a nitrogen source. In contrast, when ammonium acetate was omitted from this medium, the cells did not show any noticeable growth. Southern blotting analysis of the chromosome of Azotomonas strain ATCC13544 did not show any hybridization with a probe made of DNA sequences corresponding to the nifH gene of Azotobacter vinelandii. Based on these results, we conclude that the Azotomonas fluorescens ATCC13544 does not contain the conventional genetic information on its chromosome that is needed to utilize atmospheric nitrogen as the sole source nitrogen to support its growth.