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TouR-Mediated Effector-Independent Growth Phase-Dependent Activation of the ⁵⁴ Ptou Promoter of Pseudomonas stutzeri OX1

Dafne Solera,^1, Fabio L. G. Arenghi,^1, Tanja Woelk,¹ Enrica Galli,¹ and Paola Barbieri^2*

Dipartimento di Scienze Biomolecolari e Biotecnologie, Università degli Studi di Milano, Milan,¹ Dipartimento di Biologia Strutturale e Funzionale, Università dell'Insubria, Varese, Italy²

Received 29 April 2004/ Accepted 28 July 2004

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 phenol-responsive 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.

Present address: Department of Molecular Biology, Umeå University, S-901 87 Umeå, Sweden.

Present address: ACS Dobfar, 20067 Tribiano, Italy.

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