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Journal of Bacteriology, December 2005, p. 7931-7944, Vol. 187, No. 23
0021-9193/05/$08.00+0     doi:10.1128/JB.187.23.7931-7944.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

sinI- and expR-Dependent Quorum Sensing in Sinorhizobium meliloti{dagger}

Mengsheng Gao,1 Hancai Chen,2 Anatol Eberhard,3 Matthew R. Gronquist,4 Jayne B. Robinson,5 Barry G. Rolfe,2 and Wolfgang D. Bauer1*

Department of Horticulture and Crop Science, Ohio State University, 2021 Coffey Rd., Columbus, Ohio 43210,1 Genomic Interactions Group, Research School for Biological Sciences, Australian National University, Canberra ACT, Australia,2 Department of Microbiology, Wing Hall 359, Cornell University, Ithaca, New York 14853,3 Chemistry Department, SUNY Cortland, Cortland, New York, 13045,4 Biology Department, University of Dayton, Dayton, Ohio 454605

Received 2 July 2005/ Accepted 12 September 2005

Quorum sensing (QS) in Sinorhizobium meliloti, the N-fixing bacterial symbiont of Medicago host plants, involves at least half a dozen different N-acyl homoserine lactone (AHL) signals and perhaps an equal number of AHL receptors. The accumulation of 55 proteins was found to be dependent on SinI, the AHL synthase, and/or on ExpR, one of the AHL receptors. Gas chromatography-mass spectrometry and electrospray ionization tandem mass spectrometry identified 3-oxo-C14-homoserine lactone (3-oxo-C14-HSL), C16-HSL, 3-oxo-C16-HSL, C16:1-HSL, and 3-oxo-C16:1-HSL as the sinI-dependent AHL QS signals accumulated by the 8530 expR+ strain under the conditions used for proteome analysis. The 8530 expR+ strain secretes additional, unidentified QS-active compounds. Addition of 200 nM C14-HSL or C16:1-HSL, two of the known SinI AHLs, affected the levels of 75% of the proteins, confirming that their accumulation is QS regulated. A number of the QS-regulated proteins have functions plausibly related to symbiotic interactions with the host, including ExpE6, IdhA, MocB, Gor, PckA, LeuC, and AglE. Seven of 10 single-crossover ß-glucuronidase (GUS) transcriptional reporters in genes corresponding to QS-regulated proteins showed significantly different activities in the sinI and expR mutant backgrounds and in response to added SinI AHLs. The sinI mutant and several of the single-crossover strains were significantly delayed in the ability to initiate nodules on the primary root of the host plant, Medicago truncatula, indicating that sinI-dependent QS regulation and QS-regulated proteins contribute importantly to the rate or efficiency of nodule initiation. The sinI and expR mutants were also defective in surface swarming motility. The sinI mutant was restored to normal swarming by 5 nM C16:1-HSL.

* Corresponding author. Mailing address: Department of Plant Sciences, University of California, One Shields Ave., Davis, CA 95616. Phone: (530) 752-7732. Fax: (530) 752-4361. E-mail: bauer.7{at}osu.edu.

{dagger} Horticulture and Crop Science report 05-17.

Journal of Bacteriology, December 2005, p. 7931-7944, Vol. 187, No. 23
0021-9193/05/$08.00+0     doi:10.1128/JB.187.23.7931-7944.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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