TouR-mediated effector-independent growth-phase-dependent activation of the σ54 Ptou promoter of Pseudomonas stutzeri OX1

Transcription of the catabolic touABCDEF operon, encoding the toluene-o-xylene monooxygenase of Pseudomonas stutzeri OX1, is driven by the 54-dependent Ptou promoter, whose activity is controlled by the phenolresponsive NtrC-like activator TouR. In this paper we describe for the first time a peculiar characteristic of this system, namely, that Ptou transcription is activated in a growth phase-dependent manner in the absence of genuine effectors of the cognate TouR regulator. This phenomenon, which we named gratuitous activation, was observed in the native strain P. stutzeri OX1, as well as in a Pseudomonas putida PaW340 host harboring the reconstructed tou regulatory circuit. Regulator-promoter swapping experiments demonstrated that the presence of TouR is necessary and sufficient for imposing gratuitous activation on the Ptou promoter, as well as on other 54-dependent catabolic promoters, whereas the highly similar phenol-responsive activator DmpR is unable to activate the Ptou promoter in the absence of effectors. We show that this phenomenon is specifically triggered by carbon source exhaustion but not by nitrogen starvation. An updated model of the tou regulatory circuit is presented.