Transcriptional control of the Pseudomonas putida TOL plasmid catabolic pathways

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Abstract
TOL plasmid pWWO of Pseudomonas putida contains two operons that specify a pathway for the degradation of aromatic hydrocarbons. The upper pathway operon encodes the enzymes for the oxidation of toluene/xylenes to benzoate/toluates, and the meta‐cleavage pathway operon encodes the enzymes for the further oxidation of these compounds to Krebs cycle intermediates. Their expression is controlled by the gene products of two divergently transcribed regulatory genes, xylR and xylS. The XylR protein, which belongs to the NtrC family of regulators, is expressed from two tandem promoters and autoregulates its synthesis. XylR stimulates transcription from the xylS gene promoter (Ps) and the upper pathway operon promoter (Pu) in the presence of pathway substrates. Both promoters are a54 dependent, and Pu also requires the presence of integration host factor (IHF) for activation of transcription. Binding sites for XylR and IHF in the Pu promoter and for XylR in the Ps promoters have been defined. The XylS protein, which belongs to the AraC family of regulators, stimulates transcription from the mefa‐cleavage pathway operon promoter (Pm) in the presence of benzoates. The effector binding pocket and ONA‐binding region of XylS have been defined through the isolation of mutants that exhibit altered effector specificity and modified transcriptional patterns, respectively. Expression of the mefa‐cleavage pathway operon is also induced by xylene‐activated XylR protein via a cascade regulatory system in which this protein, in combination with σ;54, stimulates the expression from the xylS promoter. The increased concentration of XylS in turn leads to high level expression of the