Stimulation of transcription from the Pseudomonas TOL plasmid xylS gene promoter (Ps) and the upper-pathway operon promoter (Pu) is dependent on the positive regulator protein XylR activated by an effector molecule such as 3-cholorotoluene, and on RpoN, an RNA polymerase sigma factor. Mutational analysis of the Ps and Pu promoters showed that upstream activator sequences located between -110 and -218bp upstream of the main transcription initiation point are required for regulated expression from these promoters. A search for homologous nucleotide sequences in the -110 to -218bp region in Pu and Ps revealed conserved sequences that may act as putative recognition sequences for the XylR protein. Ps and Pu exhibit another well-conserved region at around -50bp, which is homologous to corresponding sites in other RpoN-dependent promoters and may constitute a binding site for integration host factor (IHF).