TheAzospirillum brasilense rpoNgene is involved in nitrogen fixation, nitrate assimilation, ammonium uptake, and flagellar biosynthesis

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Abstract
The rpoN (ntrA) gene (encoding sigma 54) of Azospirillum brasilense Sp7 was isolated by using conserved rpoN primers and the polymerase chain reaction, and its nucleotide sequence was determined. The deduced amino acid sequence of the RpoN protein was found to share a high degree of homology with other members of the sigma 54 family. Two additional open reading frames were found in the Azospirillum brasilense rpoN region, with significant similarity to equivalent regions surrounding the rpoN locus in other bacteria. An rpoN mutant of Azospirillum brasilense Sp7 was constructed by gene replacement and found to be defective in nitrogen fixation, nitrate assimilation, and ammonium uptake. Lack of ammonium uptake was also found in previously isolated Azospirillum brasilense ntrB and ntrC mutants, further supporting the role of the ntr system in this process. In addition, the rpoN mutant was found to be nonmotile, suggesting a role of RpoN in Azospirillum brasilense flagellar biosynthesis.Key words: Azospirillum brasilense, sigma factor, nitrogen fixation, ammonium assimilation, motility.