Aromatic aminotransferase activity and indoleacetic acid production in Rhizobium meliloti

Bacterial indoleacetic acid (IAA) production, which has been proposed to play a role in the Rhizobium-legume symbiosis, is a poorly understood process. Previous data have suggested that IAA biosynthesis in Rhizobium meliloti can occur through an indolepyruvate intermediate derived from tryptophan by an aminotransferase activity. To further examine this biosynthetic pathway, the aromatic aminotransferase (AAT) activity of Rhizobium meliloti 102F34 (F34) was characterized. At least four proteins were detected on nondenaturing gels of F34 protein extracts that exhibited AAT activity. All four of these AATs were constitutively produced and utilized the aromatic amino acids tryptophan, phenylalanine, and tyrosine as amino substrates. Two AATs were also capable of using aspartate. Plasmids from an F34 gene bank were identified that coded for the synthesis of at least three of these proteins, and the respective gene sequences were localized by transposon mutagenesis. Selected transposon insertions were recombined into the F34 genome to produce strains defective in two of these proteins (AAT1 and AAT2). Characterization of the mutants revealed that neither was essential for the biosynthesis of IAA in the absence of exogenous tryptophan, but that both contributed to IAA biosynthesis when high levels of exogenous tryptophan were present. AAT1 and AAT2 were also not required for the production of a minimal level of aromatic amino acids, but both were able to scavenge nitrogen from the aromatic amino acids during nitrogen deprivation. Neither AAT1 nor AAT2 was essential for symbiosis with alfalfa.