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J. Biol. Chem., Vol. 281, Issue 11, 7102-7109, March 17, 2006

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Effector-Repressor Interactions, Binding of a Single Effector Molecule to the Operator-bound TtgR Homodimer Mediates Derepression^*

From the Department of Biochemistry and Molecular and Cellular Biology of Plants, Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas, Apartado de Correos 419, E-18008 Granada, Spain

The RND family transporter TtgABC and its cognate repressor TtgR from Pseudomonas putida DOT-T1E were both shown to possess multidrug recognition properties. Structurally unrelated molecules such as chloramphenicol, butyl paraben, 1,3-dihydroxynaphthalene, and several flavonoids are substrates of TtgABC and activate pump expression by binding to the TtgR-operator complex. Isothermal titration calorimetry was employed to determine the thermodynamic parameters for the binding of these molecules to TtgR. Dissociation constants were in the range from 1 to 150 µM, the binding stoichiometry was one effector molecule per dimer of TtgR, and the process was driven by favorable enthalpy changes. Although TtgR exhibits a large multidrug binding profile, the plant-derived compounds phloretin and quercetin were shown to bind with the highest affinity (K_D of around 1 µM), in contrast to other effectors (chloramphenicol and aromatic solvents) for which exhibited a more reduced affinity. Structure-function studies of effectors indicate that the presence of aromatic rings as well as hydroxyl groups are determinants for TtgR binding. The binding of TtgR to its operator DNA does not alter the protein effector profile nor the effector binding stoichiometry. Moreover, we demonstrate here for the first time that the binding of a single effector molecule to the DNA-bound TtgR homodimer induces the dissociation of the repressor-operator complex. This provides important insight into the molecular mechanism of effector-mediated derepression.

Received for publication, October 12, 2005 , and in revised form, January 5, 2006.

^* This work was supported in part by European Community Grant QLRT-2001-01923 (to J. L. R.) and Grant RGY0021/2002 from the Human Frontier Science Program and Grant BFU-2004-00045/BMC from the Ministerio de Educación y Ciencia (Spain) (to M.-T. G.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ Recipient of a fellowship from the Junta de Andalucía, Spain.

² To whom correspondence should be addressed. Tel.: 34-958181600; Fax: 34-958129600; E-mail: maritrini.gallegos{at}eez.csic.es.

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