Using a transposon carrying a promoterless lux operon to generate transcriptional fusions by insertional mutagenesis, we have identified a Pseudomonas putida gene with increased expression in the presence of corn root exudates. Expression of the transcriptional fusion, induced by the amino acid lysine, was detected in P. putida in the rhizosphere of plants as well as in response to seed exudates. The mutant was unable to grow on lysine or delta-aminovalerate as carbon sources, which indicates that the affected function is involved in the pathway for lysine catabolism. However, the mutant strain grew with glutaric acid, the product of delta-aminovalerate metabolism via glutaric acid semialdehyde, as a C source. The translated sequence of the interrupted gene showed high levels of similarity with aminotransferases. These sets of data suggest that the product of this gene has delta-aminovalerate aminotransferase activity. This is the first direct genetic evidence correlating a DNA sequence with such activity in Pseudomonadaceae.