In Rhizobium meliloti, an aspartate aminotransferase (AspAT) encoded within a 7.3-kb HindIII fragment was previously shown to be required for symbiotic nitrogen fixation and aspartate catabolism (V. K. Rastogi and R.J. Watson, J. Bacteriol. 173:2879-2887, 1991). A gene coding for an aromatic aminotransferase located within an 11-kb HindIII fragment was found to complement the AspAT deficiency when overexpressed. The genes encoding these two aminotransferases, designated aatA and tatA, respectively, have been localized by subcloning and transposon Tn5 mutagenesis. Sequencing of the tatA gene revealed that it encodes a protein homologous to an Escherichia coli aromatic aminotransferase and most of the known AspAT enzymes. However, sequencing of the aatA gene region revealed two overlapping open reading frames, neither of which encoded an enzyme with homology to the typical AspATs. Polymerase chain reaction was used to selectively generate one of the candidate sequences for subcloning. The cloned fragment complemented the original nitrogen fixation and aspartate catabolism defects and was shown to encode an AspAT with the expected properties. Sequence analysis showed that the aatA protein has homology to AspATs from two thermophilic bacteria and the eukaryotic tyrosine aminotransferases. These aminotransferases form a distinct class in which only 13 amino acids are conserved in comparison with the well-known AspAT family. DNA homologous to the aatA gene was found to be present in Agrobacterium tumefaciens and other rhizobia but not in Klebsiella pneumoniae or E. coli.