The genes of TOL plasmid that specify catabolism of toluene and xylenes are organized in four transcriptional units: the upper-operon for conversion of toluene/xylenes into benzoate/alkylbenzoates; the -operon , which encodes the enzymes for further conversion of these compounds into Krebs cycle intermediates; and and , 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 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 σ70- or σS-containing RNA polymerase (the loop). The gene is also expressed constitutively. The XylR protein, which in the absence of effectors 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 σ54-containing RNA polymerase. Once activated, the XylR protein also stimulates transcription from the Ps1 promoter of without interfering with expression from Ps2. This process is assisted by the HU protein and is mediated by σ54-containing RNA polymerase. As a consequence of hyperexpression of the gene, the XylS protein is hyperproduced and stimulates transcription from Pm even in the absence of effectors (the cascade loop). The two σ54-dependent promoters are additionally subject to global (catabolite repression) control.


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  • Article Type: Review Article
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