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Journal of Bacteriology, February 2000, p. 599-606, Vol. 182, No. 3
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Environmental Regulation of Exopolysaccharide Production in Sinorhizobium meliloti

Kiprian E. Mendrygal and Juan E. González*

Department of Molecular and Cell Biology, University of Texas at Dallas, Richardson, Texas 75083-0688

Received 26 August 1999/Accepted 29 October 1999

Exopolysaccharide production by Sinorhizobium meliloti is required for invasion of root nodules on alfalfa and successful establishment of a nitrogen-fixing symbiosis between the two partners. S. meliloti wild-type strain Rm1021 requires production of either succinoglycan, a polymer of repeating octasaccharide subunits, or EPS II, an exopolysaccharide of repeating dimer subunits. The reason for the production of two functional exopolysaccharides is not clear. Earlier reports suggested that low-phosphate conditions stimulate the production of EPS II in Rm1021. We found that phosphate concentrations determine which exopolysaccharide is produced by S. meliloti. The low-phosphate conditions normally found in the soil (1 to 10 µM) stimulate EPS II production, while the high-phosphate conditions inside the nodule (20 to 100 mM) block EPS II synthesis and induce the production of succinoglycan. Interestingly, the EPS II produced by S. meliloti in low-phosphate conditions does not allow the invasion of alfalfa nodules. We propose that this invasion phenotype is due to the lack of the active molecular weight fraction of EPS II required for nodule invasion. An analysis of the function of PhoB in this differential exopolysaccharide production is presented.

* Corresponding author. Mailing address: FO 3.1, Department of Molecular and Cellular Biology, University of Texas at Dallas, Richardson, TX 75083-0688. Phone: (972) 883-2526. Fax: (630) 604-3093. E-mail: jgonzal{at}utdallas.edu.

Journal of Bacteriology, February 2000, p. 599-606, Vol. 182, No. 3
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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