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J. Biol. Chem., Vol. 280, Issue 44, 36802-36808, November 4, 2005

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Structural and Genetic Analyses Reveal a Key Role in Prophage Excision for the TorI Response Regulator Inhibitor^*

Latifa ElAntak1, Mireille Ansaldi1, Françoise Guerlesquin, Vincent Méjean, and Xavier Morelli2

From the Unité de Bioénergétique et Ingénierie des Protéines and the Laboratoire de Chimie Bactérienne, IBSM-CNRS, 31 chemin Joseph Aiguier, 13402 Marseille Cedex 20, France

TorI (Tor inhibition protein) has been identified in Escherichia coli as a protein inhibitor acting through protein-protein interaction with the TorR response regulator. This interaction, which does not interfere with TorR DNA binding activity, probably prevents the recruitment of RNA polymerase to the torC promoter. In this study we have solved the solution structure of TorI, which adopts a prokaryotic winged-helix arrangement. Despite no primary sequence similarity, the three-dimensional structure of TorI is highly homologous to the Xis, Mu bacteriophage repressor (MuR-DBD), and transposase (MuA-DBD) structures. We propose that the TorI protein is the structural missing link between the Xis and MuR proteins. Moreover, in vivo assays demonstrated that TorI plays an essential role in prophage excision. Heteronuclear NMR experiments and site-directed mutagenesis studies have pinpointed out key residues involved in the DNA binding activity of TorI. Our findings suggest that TorI-related proteins identified in various pathogenic bacterial genomes define a new family of atypical excisionases.

Received for publication, July 8, 2005 , and in revised form, August 1, 2005.

The atomic coordinates and structure factors (code 1Z4H) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

^* This work was supported by the CNRS and the Université de la Méditerranée. 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 Both authors contributed equally to this work.

² To whom correspondence should be addressed. Tel.: 33-491-164-647; Fax: 33-491-164-578; E-mail: morelli{at}ibsm.cnrs-mrs.fr.

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