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.