Aromatic amino acid aminotransferase activity and indole‐3‐acetic acid production by associative nitrogen‐fixing bacteria

In this work, we report the detection of aromatic amino acid aminotransferase (AAT) activity from cell‐free crude extracts of nine strains of N 2 ‐fixing bacteria from three genera. Using tyrosine as substrate, AAT activity ranged in specific activity from 0.084 to 0.404 μmol min −1 mg −1 . When analyzed under non‐denaturating PAGE conditions; and using tryptophan, phenylalanine, tyrosine, and histidine as substrates Pseudomonas stutzeri A15 showed three isoforms with molecular mass of 46, 68 and 86 kDa, respectively; Azospirillum strains displayed two isoforms which molecular mass ranged from 44 to 66 kDa and Gluconacetobacter strains revealed one enzyme, which molecular mass was estimated to be much more higher than those of Azospirillum and P. stutzeri strains. After SDS–PAGE, some AAT activity was lost, indicating a differential stability of proteins. All the strains tested produced IAA, especially with tryptophan as precursor. Azospirillum strains produced the highest concentrations of IAA (16.5–38 μg IAA/mg protein), whereas Gluconacetobacter and P. stutzeri strains produced lower concentrations of IAA ranging from 1 to 2.9 μg/mg protein in culture medium supplemented with tryptophan. The IAA production may enable bacteria promote a growth‐promoting effect in plants, in addition to their nitrogen fixing ability.