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Journal of Bacteriology, September 2002, p. 5067-5076, Vol. 184, No. 18
0021-9193/02/$04.00+0     DOI: 10.1128/JB.184.18.5067-5076.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

A LuxR Homolog Controls Production of Symbiotically Active Extracellular Polysaccharide II by Sinorhizobium meliloti

Brett J. Pellock,1 Max Teplitski,2 Ryan P. Boinay,1 W. Dietz Bauer,2 and Graham C. Walker1*

Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139,1 Department of Horticulture and Crop Science, Plant Molecular Biology and Biotechnology Program, Ohio State University, Columbus, Ohio 432102

Received 26 December 2001/ Accepted 2 June 2002

Production of complex extracellular polysaccharides (EPSs) by the nitrogen-fixing soil bacterium Sinorhizobium meliloti is required for efficient invasion of root nodules on the host plant alfalfa. Any one of three S. meliloti polysaccharides, succinoglycan, EPS II, or K antigen, can mediate infection thread initiation and extension (root nodule invasion) on alfalfa. Of these three polysaccharides, the only symbiotically active polysaccharide produced by S. meliloti wild-type strain Rm1021 is succinoglycan. The expR101 mutation is required to turn on production of symbiotically active forms of EPS II in strain Rm1021. In this study, we have determined the nature of the expR101 mutation in S. meliloti. The expR101 mutation, a spontaneous dominant mutation, results from precise, reading frame-restoring excision of an insertion sequence from the coding region of expR, a gene whose predicted protein product is highly homologous to the Rhizobium leguminosarum bv. viciae RhiR protein and a number of other homologs of Vibrio fischeri LuxR that function as receptors for N-acylhomoserine lactones (AHLs) in quorum-sensing regulation of gene expression. S. meliloti ExpR activates transcription of genes involved in EPS II production in a density-dependent fashion, and it does so at much lower cell densities than many quorum-sensing systems. High-pressure liquid chromatographic fractionation of S. meliloti culture filtrate extracts revealed at least three peaks with AHL activity, one of which activated ExpR-dependent expression of the expE operon.

* Corresponding author. Mailing address: Department of Biology 68-633, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139. Phone: (617) 253-6716. Fax: (617) 253-2643. E-mail: gwalker{at}mit.edu.

Journal of Bacteriology, September 2002, p. 5067-5076, Vol. 184, No. 18
0021-9193/02/$04.00+0     DOI: 10.1128/JB.184.18.5067-5076.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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