TouR-Mediated Effector-Independent Growth Phase-Dependent Activation of the σ54 Ptou Promoter of Pseudomonas stutzeri OX1

TouR-Mediated Effector-Independent Growth Phase-Dependent Activation of the σ 54 P tou Promoter of Pseudomonas stutzeri OX1 Dafne Solera 1 , † , Fabio L. G. Arenghi 1 , ‡ , Tanja Woelk 1 , Enrica Galli 1 , and Paola Barbieri 2 , * 1 Dipartimento di Scienze Biomolecolari e Biotecnologie, Università degli Studi di Milano, Milan 2 Dipartimento di Biologia Strutturale e Funzionale, Università dell'Insubria, Varese, Italy ABSTRACT Transcription of the catabolic touABCDEF operon, encoding the toluene- o -xylene monooxygenase of Pseudomonas stutzeri OX1, is driven by the σ 54 -dependent P tou promoter, whose activity is controlled by the phenol-responsive NtrC-like activator TouR. In this paper we describe for the first time a peculiar characteristic of this system, namely, that P tou 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 P tou promoter, as well as on other σ 54 -dependent catabolic promoters, whereas the highly similar phenol-responsive activator DmpR is unable to activate the P tou 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.