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J. Biol. Chem., Vol. 280, Issue 22, 21498-21505, June 3, 2005

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Antagonistic Role of H-NS and GadX in the Regulation of the Glutamate Decarboxylase-dependent Acid Resistance System in Escherichia coli^*

Mara Giangrossi, Stefano Zattoni, Angela Tramonti¶||, Daniela De Biase||, and Maurizio Falconi**

From the Laboratorio di Genetica, Dipartimento di Biologia MCA, Università di Camerino, 62032 Camerino (MC) and the ^¶Istituto di Biologia e Patologia Molecolari del Consiglio Nazionale delle Ricerche and the ^||Dipartimento di Scienze Biochimiche "A. Rossi Fanelli," Università di Roma La Sapienza, 00185 Roma, Italy

One of the most efficient systems of acid resistance in Escherichia coli, the gad system, is based on the coordinated action of two isoforms of glutamate decarboxylase (GadA and GadB) and of a specific glutamate/-aminobutyrate antiporter (GadC). The gadA/BC genes, activated in response to acid stress and in stationary phase cells, are subjected to complex circuits of regulation involving ⁷⁰, ^S, cAMP receptor protein, H-NS, EvgAS, TorRS, GadE, GadX, GadW, and YdeO. Herein, we provide evidence that the nucleoid-associated protein H-NS directly functions as repressor of gadA, one of the structural genes, and gadX, a regulatory gene encoding one of the primary activators of the gad system. Band shift and DNase I footprints reveal that H-NS indeed binds to specific sites in the promoter regions of gadA and gadX and represses the transcription of these genes both in an in vitro system and in vivo. Moreover, we show that a maltose-binding protein MalE-GadX fusion is able to stimulate the promoter activity of gadA/BC, thus indicating that GadX is by itself able to up-regulate the gad genes and that a functional competition between H-NS and GadX takes place at the gadA promoter. Altogether, our results indicate that H-NS directly inhibits gadA and gadX transcription and, by controlling the intracellular level of the activator GadX, indirectly affects the expression of the whole gad system.

Received for publication, November 24, 2004 , and in revised form, March 22, 2005.

^* This work was supported by grants from MIUR (to M. F. and D. D. B.) and from the Istituto Pasteur-Fondazione Cenci Bolognetti (to D. D. B.). 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.

Both authors contributed equally to this work.

^** To whom correspondence should be addressed. Tel.: 39-737-403265; Fax: 39-737-403248; E-mail: maurizio.falconi{at}unicam.it.

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