The phaseolotoxin-resistant ornithine carbamoyltransferase (ROCT) and phaseolotoxin are produced by Pseudomonas syringae pv. phaseolicola at 18 degrees C but not at 28 degrees C. At 28 degrees C, the pathogen produces a protein(s) that binds (in vitro) to a 485-bp fragment (thermoregulatory region, TRR) from a heterologous clone from the pathogen genomic library, which in multiple copies overrides thermoregulation of phaseolotoxin production in wild-type cells (K. B. Rowley, D. E. Clements, M. Mandel, T. Humphreys, and S. S. Patil, Mol. Microbiol. 8:625-635, 1993). We report here that DNase I protection analysis of the 485-bp fragment shows that a single site is protected from cleavage by the protein in the 28 degrees C extract and that this site contains two repeats of a core motif G/C AAAG separated by a 5-bp spacer. Partially purified binding protein forms specific complexes with a synthetic oligonucleotide containing four tandem repeats of this motif. A 492-bp upstream fragment from argK encoding ROCT also forms specific complexes with the protein in the 28 degrees C crude extract, and a 260-bp subfragment from the TRR containing the binding site cross competes with the argk fragment, indicating that the same protein binds to nucleotides in both fragments. DNase I protection analysis of the fragment from argK revealed four separate protected sequence elements, with element III containing half of the core motif sequence (CTTTG), and the other elements containing similar sequences. Gel shift assays were done with DNA fragments from which one or all of the sites were removed as competitor DNAs against the argK probe. The results of these experiments confirmed that the binding sites (in argK) are necessary for the protein to bind to the argK fragment in a specific manner. Taken together, the results of studies presented here suggest that in cells of P. syringae pv. phaseolicola grown at high temperature argK may be negatively regulated by the protein produced at this temperature.