The differential response of the Pben promoter of Pseudomonas putidamt-2 to BenR and XylS prevents metabolic conflicts in m-xylene biodegradation

Pseudomonas putidamt-2 encompasses two alternative and potentially conflicting routes for benzoate metabolism, one meta pathway encoded by xyl genes of the pWW0 plasmid and mastered by the Pm promoter and XylS, and one chromosomally encoded ortho pathway initiated by Pben and the BenR protein. Any cross-activation of Pben promoter by XylS ought to cause a metabolic conflict during the degradation of m-xylene because 3-methylbenzoate (3MBz) generated as an intermediate can be channelled through the ortho pathway and produce toxic dead-end metabolites. The activation of Pben by XylS was revisited using both reporter technology and tiling arrays targeted to the sequences of interest around messenger RNA initiation of both Pben and Pm promoters. Analysis of supersensitive luxCDABE fusions, inspection of xylX versus benA transcripts and growth tests of benR mutants indicated that the natural expression ranges of XylS under various conditions are insufficient to cause a significant cross-regulation of Pben whether cells face endogenous or exogenous 3MBz. This seems to stem from the nature of the operators for binding either transcriptional factor, which in the case of the Pben promoter of P.putidamt-2 appear to have evolved for avoiding a strong interaction with XylS.