The phytohormone indole-3-acetic acid (IAA) accumulates in the culture medium of the plant growth-promoting bacterium Pseudomonas putida GR12-2 only when grown in the presence of exogenous tryptophan, suggesting that expression of indolepyruvate decarboxylase, a key enzyme in the IAA biosynthesis pathway in this bacterium, may be regulated by tryptophan. To test this hypothesis, we isolated the promoter region for the ipdc gene encoding indolepyruvate decarboxylase by inverse polymerase chain reaction (PCR) and inserted it upstream of the bioluminescent reporter gene luxAB on a plasmid in P. putida GR12-2. Activity of the ipdc promoter, measured by quantifying light production, increased fivefold in the presence of L-tryptophan, confirming that ipdc expression is induced by tryptophan. In addition, transcription of ipdc is regulated by the stationary phase sigma factor RpoS: the ipdc promoter contains a sequence similar to the RpoS recognition sequence, and transformation of P. putida GR12-2 with a plasmid carrying rpoS under the control of a constitutive promoter induced promoter activity before the onset of stationary phase when RpoS is not normally produced and prolonged a higher level of transcription at the later stages of the cell cycle.