Six of the genes encoding enzymes of the beta-ketoadipate pathway for benzoate and 4-hydroxybenzoate degradation in Pseudomonas putida are organized into at least three separate transcriptional units. As an initial step to defining this pca regulon at the molecular level, lacZ fusions were made with the pcaI and pcaJ genes, which encode the two subunits of beta-ketoadipate:succinyl-coenzyme A transferase, the enzyme catalyzing the next-to-last step in the beta-ketoadipate pathway. Fusion analyses showed that pcaI and pcaJ constitute an operon which requires beta-ketoadipate or its nonmetabolizable analog, adipate, as well as the pcaR regulatory gene for induction. The pcaIJ promoter is likely to be a sigma 70-type promoter; it has a sigma 70-type consensus sequence and did not require the alternative sigma factor, RpoN, for induction. Deletion analysis of the promoter region of a pcaI-lacZ transcriptional fusion indicated that no specific DNA sequences upstream of the -35 region were required for full induction. This implies that the binding site for the activator protein, PcaR, is unusually close to the transcriptional start site of pcaIJ.