Activation of the Pseudomonas TOL plasmid upper pathway operon. Identification of binding sites for the positive regulator XylR and for integration host factor protein.

Expression of the Pseudomonas putida TOL plasmid upper pathway operon requires a promoter that belongs to the -12/-24 class. Stimulation of transcription from this promoter is positively controlled by the effector-activated XylR protein and requires a form of RNA-polymerase holoenzyme containing the RpoN-encoded sigma factor, sigma 54. XylR-dependent stimulation of transcription from the Pseudomonas TOL upper pathway promoter was examined using deletions, insertions, and in vivo dimethyl sulfate footprinting. Two upstream activator sequences were identified in the -160 (UAS1) and -130 (UAS2) regions. Deletion of these two regions abolished transcription activation, although conservation of the UAS2 element alone allowed limited transcription stimulation. Separation of UAS1 from UAS2 by half a turn or a full turn significantly reduced XylR stimulation of transcription from the upper pathway operon promoter. An inverted repeated ATTTGN2CAAAT (where N is any nucleoside), which most likely represented the XylR recognition sequence, was identified. Binding of XylR was observed in vivo in the absence of effector, but changes in the binding pattern were induced in the presence of m-methylbenzyl alcohol, a XylR effector. In vivo footprinting analysis revealed that changes in the methylation pattern of G and T also occurred in the -50 to -90 region, which is probably occupied by integration host factor (IHF) protein. IHF was required for maximal expression from the TOL upper pathway operon promoter in Escherichia coli. Separation of the IHF site from UAS2 by a full helix turn did not significantly affect stimulation of transcription, which is consistent with this region playing a conformational role, rather than a regulatory one, in promoter function.