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J. Bacteriol. doi:10.1128/JB.01044-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

MECHANISMS OF TRANSCRIPTION ACTIVATION EXERTED BY GadX AND GadW AT THE gadA AND gadBC GENE PROMOTERS OF THE GLUTAMATE-BASED ACID RESISTANCE SYSTEM IN ESCHERICHIA COLI

Istituto di Biologia e Patologia Molecolari, CNR and Istituto Pasteur - Fondazione Cenci Bolognetti, Dipartimento di Scienze Biochimiche "A. Rossi Fanelli", Università di Roma La Sapienza, Roma, Italy, Molecular Immunology Unit, Chiron Vaccines, Siena, Italy and Dipartimento di Biologia, Università di Bologna, Bologna, Italy

^* To whom correspondence should be addressed. Email: daniela.debiase{at}uniroma1.it.

	Abstract

In Escherichia coli the gad system protects the cell from the extreme acid stress encountered during transit through the host stomach. The structural genes gadA, gadB and gadC encode two glutamate decarboxylase isoforms and a glutamate/GABA antiporter, respectively. Glutamate decarboxylation involves both proton consumption and production of GABA, a neutral compound which is finally exported via the GadC antiporter. Regulation of gadA and gadBC transcription is very complex, involving several circuits controlling expression under different growth phase, media, and pH conditions. In this study we show that the AraC-like activators GadX and GadW share the same 44-bp binding site within the gadA and gadBC regulatory regions. These common binding sites are centered at 110.5 bp and 220.5 bp upstream of the transcriptional startpoints of the gadA and gadBC genes, respectively. At the gadA promoter this regulatory element overlaps with one of the binding sites of the repressor H-NS. The DNA of the gadBC promoter is endowed with an intrinsic bend which is centered at position -121. These findings combined with transcriptional regulation studies may account for two different mechanisms of transcriptional activation exerted by GadX and GadW at the two promoters under study. We speculate that while at the gadA promoter GadX and GadW activate transcription by displacing H-NS via an anti-repressor mechanism, at the gadBC promoter the mechanism of activation involves looping of the DNA sequence between the promoter and the activator binding site.

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