Integration of the quorum-sensing system in the regulatory networks controlling virulence factor synthesis in Erwinia chrysanthemi

Authors

  • Sylvie Reverchon,

    1. Laboratoire de Génétique Moléculaire des Microorganismes et des Interactions Cellulaires, UMR-CNRS 5577, INSA, Bat 406, 20, Avenue A. Einstein, F-69621 Villeurbanne Cedex, France.,
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  • Marie Louise Bouillant,

    1. Laboratoire d'Ecologie Microbienne, UMR-CNRS 5557, Université Claude Bernard, Bat 741, 43 Boulevard du 11 Novembre, F-69622 Villeurbanne Cedex, France.,
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  • George Salmond,

    1. Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB2 1QW, UK.
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  • William Nasser

    1. Laboratoire de Génétique Moléculaire des Microorganismes et des Interactions Cellulaires, UMR-CNRS 5577, INSA, Bat 406, 20, Avenue A. Einstein, F-69621 Villeurbanne Cedex, France.,
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Sylvie Reverchon , E-mail lgmm@cismibm.univ-lyon1.fr; Tel. (472) 43 80 88; Fax (472) 43 87 14.

Abstract

The expI–expR locus drives a quorum-sensing system in the phytopathogenic bacterium, Erwinia chrysanthemi. Purified ExpR, an N-acyl homoserine lactone-responsive regulatory protein, binds to the promoter/operator region of the expI and expR genes. DNase I footprinting experiments showed that ExpR protects the regions between −66 and −40 from the P1 transcription initiation site of expI and between −54 and −18 from the expR transcription initiation site P1. The protected region overlaps the two expR promoters, P1 and P2, suggesting that ExpR exerts a negative control on its own gene expression. This assertion is reinforced by the fact that the addition of OHHL dissociates the ExpR–expR DNA complex. In contrast, the location of the ExpR binding site on the expI gene suggests an activator function, as reported for the pel genes. Moreover, ExpR is able to induce DNA bending. In vivo and in vitro studies revealed that CRP functions as an activator of expR expression, but as a repressor of expI transcription. A second level of control of expR and expI occurs through the PecS repressor, a regulator of pectinase synthesis. PecS represses expI expression, while ExpR activates pecS transcription, suggesting the existence of a mutual control between pecS and the expIexpR system in E. chrysanthemi. Regulation of pectinase synthesis in soft rot Erwinia appears to be a complex network of multiple cross-acting regulatory elements. A model that integrates these regulatory elements is proposed.

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