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Journal of Bacteriology, May 2009, p. 3059-3067, Vol. 191, No. 9
0021-9193/09/$08.00+0     doi:10.1128/JB.01650-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

The cin and rai Quorum-Sensing Regulatory Systems in Rhizobium leguminosarum Are Coordinated by ExpR and CinS, a Small Regulatory Protein Coexpressed with CinI{triangledown}

Anne Edwards,1 Marijke Frederix,1 Florence Wisniewski-Dyé,2 Jacob Jones,1 Angeles Zorreguieta,3 and J. Allan Downie1*

John Innes Centre, Colney, Norwich NR4 7UH, United Kingdom,1 Université Lyon 1, CNRS UMR 5557 Ecologie Microbienne, 69622 Villeurbanne, France,2 Fundación Instituto Leloir, IIBBA-CONICET, FCEyN, University of Buenos Aires, Patricias Argentinas 435, Buenos Aires, Argentina3

Received 20 November 2008/ Accepted 23 February 2009

To understand how the Rhizobium leguminosarum raiI-raiR quorum-sensing system is regulated, we identified mutants with decreased levels of RaiI-made N-acyl homoserine lactones (AHLs). A LuxR-type regulator, ExpR, is required for raiR expression, and RaiR is required to induce raiI. Since raiR (and raiI) expression is also reduced in cinI and cinR quorum-sensing mutants, we thought CinI-made AHLs may activate ExpR to induce raiR. However, added CinI-made AHLs did not induce raiR expression in a cinI mutant. The reduced raiR expression in cinI and cinR mutants was due to lack of expression of cinS immediately downstream of cinI. cinS encodes a 67-residue protein, translationally coupled to CinI, and cinS acts downstream of expR for raiR induction. Cloned cinS in R. leguminosarum caused an unusual collapse of colony structure, and this was delayed by mutation of expR. The phenotype looked like a loss of exopolysaccharide (EPS) integrity; mutations in cinI, cinR, cinS, and expR all reduced expression of plyB, encoding an EPS glycanase, and mutation of plyB abolished the effect of cloned cinS on colony morphology. We conclude that CinS and ExpR act to increase PlyB levels, thereby influencing the bacterial surface. CinS is conserved in other rhizobia, including Rhizobium etli; the previously observed effect of cinI and cinR mutations decreasing swarming in that strain is primarily due to a lack of CinS rather than a lack of CinI-made AHL. We conclude that CinS mediates quorum-sensing regulation because it is coregulated with an AHL synthase and demonstrate that its regulatory effects can occur in the absence of AHLs.

* Corresponding author. Mailing address: John Innes Centre, Norwich Research Park, Colney, Norwich NR4 7 UH, United Kingdom. Phone: 44-1603-450207. Fax: 44-1603-450045. E-mail: allan.downie{at}bbsrc.ac.uk

{triangledown} Published ahead of print on 6 March 2009.

Journal of Bacteriology, May 2009, p. 3059-3067, Vol. 191, No. 9
0021-9193/09/$08.00+0     doi:10.1128/JB.01650-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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