Transcriptional control of the Pseudomonas tol plasmid catabolic operons is achieved through an interplay of host factors and plasmid-encoded regulators

The xyl genes of Pseudomonas putida TOL plasmid that specify catabolism of toluene and xylenes are organized in four transcriptional units: the upper-operon xylUWCAMBN for conversion of toluene/xylenes into benzoate/alkylbenzoates; the meta-operon xylXYZLTEGFJQKIH, which encodes the enzymes for further conversion of these compounds into Krebs cycle intermediates; ana xylS and xylR, which are involved in transcriptional control. The XylS and XylR proteins are members of the XylS/AraC and NtrC families, respectively, of transcriptional regulators. The xylS gene is constitutively expressed at a low level from the Ps2 promoter. The XylS protein is activated by interaction with alkylbenzoates, and this active form stimulates transcription from Pm by sigma(70)- or sigma(S)-containing RNA polymerase (the meta loop). The xylR gene is also expressed constitutively. The XylR protein, which in the absence of effecters binds in a nonactive form to target DNA sequences, is activated by aromatic hydrocarbons and ATP; it subsequently undergoes multimerization and structural changes that result in stimulation of transcription from Pu of the upper operon. This latter process is assisted by the IHF protein and mediated by sigma(54)-containing RNA polymerase. Once activated, the XylR protein also stimulates transcription from the Ps1 promoter of xylS without interfering with expression from Ps2. This process is assisted by the HU protein and is mediated by sigma(54)-containing RNA polymerase. As a consequence of hyperexpression of the xylS gene, the XylS protein is hyperproduced and stimulates transcription from Pm even in the absence of effecters (the cascade loop). The two sigma(54)-dependent promoters are additionally subject to global (catabolite repression) control.