Expression of the regulatory gene xylS on the TOL plasmid is positively controlled by the xylR gene product.

The regulatory gene xylS on the TOL plasmid of Pseudomonas putida activates the transcription of the xylDLEGF operon for the m-toluate-degrading pathway in the presence of m-toluate. The gene also activates the transcription of the same operon in the presence of m-xylene or m-methylbenzyl alcohol, but for this activation another regulatory gene, xylR, is required. In this study we examined the xylS expression by determining the mRNA by reverse transcriptase mapping and by monitoring the enzyme activity of the xylE gene product, which was expressed under the control of the xylS promoter. The results of the above experiments provide evidence that xylR positively controls the transcription of xylS in the presence of m-xylene or m-methylbenzyl alcohol. The xylS product thus amplified may in turn activate the xylDLEGF operon. The nucleotide sequence of the xylS promoter resembles that of the promoter of the xylCAB operon for the m-xylene-degrading pathway, which is also activated by xylR in the presence of m-xylene or m-methylbenzyl alcohol. In addition, we have demonstrated that the expression of xylR is negatively controlled by its own product. On the basis of these findings, we propose a revised model for the regulation of expression of xyl genes on the TOL plasmid.