The TrpI protein belongs to the LysR-family of procaryotic regulatory proteins. Members of this family share a characteristic similarity of their N-terminal amino acid sequences, and many of them are activators of divergently transcribed genes or operons. In Pseudomonas aeruginosa, the genes for tryptophan synthase, trpBA, are regulated by indoleglycerol phosphate (InGP) and TrpI. We demonstrate here that in the absence of InGP, the binding site of TrpI is located in the -52 to -77 region of the trpBA promoter; in the presence of InGP, the binding region is extended to the -32 region. In addition, two major, slow moving protein-DNA complexes are seen in gel retardation assays: the faster moving complex is formed in the absence of InGP and the amount of the slower moving complex is greatly enhanced in the presence of InGP. These results suggest that the binding of a second TrpI protein molecule, promoted by InGP, plays a crucial role in activating the expression of the trpBA gene pair.