XylR is a transcriptional regulatory protein that comes from the TOL Plasmid pWW0 of Pseudomonas putida [1][2]. This protein is able to bind some BTX (Benzene, Toluene, Xylene) compounds. This binding will trigger a conformational change in XylR and enable it to bind DNA at the Pu promoter involved in a pathway of metabolic reactions including BTEX degradation(3).

XylR is composed of 4 domains : from N terminal to C terminal there is the sensor of toluene-like compounds, the linker domain, the dimerization domain that contains an ATPase and the DNA binding domain(figure 1) [4].

The change in XylR conformation upon BTEX compound binding enables it to dimerize and bind an enhancer sequence upstream of the genes responsible for BTEX degradation. Then, two XylR dimers form a tetramer with the interaction of domains C and ATP [5]. The activation of the RNA polymerase is completed with the help of a integrated host factor IHF (figure 2). To finish the DNA duplex is opened by RNA polymerase and RNA elongation starts when the sigma-54 falls off the core RNA polymerase [4][5] (figure 2).

References :

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1. Abril, M. A., Michan, C., Timmis, K. N., & Ramos, J. (1989). Regulator and enzyme specificities of the TOL plasmid-encoded upper pathway for degradation of aromatic hydrocarbons and expansion of the substrate range of the pathway.Journal of bacteriology, 171(12), 6782-6790

2. Fernández, S., Lorenzo, V., and Pérez-Martin, J. (1995). Activation of the transcriptional regulator XylR of Pseudomonas putida by release of repression between functional domains. Molecular Microbiology 16, 205–213.

3. Gerischer, U. (2002). Specific and global regulation of genes associated with the degradation of aromatic compounds in bacteria. Journal of molecular microbiology and biotechnology, 4(2), 111- 122.

4. Salto, R., Delgado, A., Michán, C., Marqués, S., & Ramos, J. L. (1998). Modulation of the function of the signal receptor domain of XylR, a member of a family of prokaryotic enhancer-like positive regulators. Journal of bacteriology,180(3), 600-604.

5. Tropel, D., & Van Der Meer, J. R. (2004). Bacterial transcriptional regulators for degradation pathways of aromatic compounds. Microbiology and Molecular Biology Reviews, 68(3), 474-500.