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Journal of Bacteriology, November 2009, p. 6988-7000, Vol. 191, No. 22
0021-9193/09/$08.00+0     doi:10.1128/JB.00466-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

The rkp-1 Cluster Is Required for Secretion of Kdo Homopolymeric Capsular Polysaccharide in Sinorhizobium meliloti Strain Rm1021{triangledown} ,{dagger}

Maike G. Müller,1 Lennart S. Forsberg,2 and David H. Keating1*

Department of Microbiology and Immunology, Loyola University Chicago, Maywood, Illinois 60153,1 Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 306022

Received 4 April 2009/ Accepted 31 August 2009

Under conditions of nitrogen stress, leguminous plants form symbioses with soil bacteria called rhizobia. This partnership results in the development of structures called root nodules, in which differentiated endosymbiotic bacteria reduce molecular dinitrogen for the host. The establishment of rhizobium-legume symbioses requires the bacterial synthesis of oligosaccharides, exopolysaccharides, and capsular polysaccharides. Previous studies suggested that the 3-deoxy-D-manno-oct-2-ulopyranosonic acid (Kdo) homopolymeric capsular polysaccharide produced by strain Sinorhizobium meliloti Rm1021 contributes to symbiosis with Medicago sativa under some conditions. However, a conclusive symbiotic role for this polysaccharide could not be determined due to a lack of mutants affecting its synthesis. In this study, we have further characterized the synthesis, secretion, and symbiotic function of the Kdo homopolymeric capsule. We showed that mutants lacking the enigmatic rkp-1 gene cluster fail to display the Kdo capsule on the cell surface but accumulate an intracellular polysaccharide of unusually high Mr. In addition, we have demonstrated that mutations in kdsB2, smb20804, and smb20805 affect the polymerization of the Kdo homopolymeric capsule. Our studies also suggest a role for the capsular polysaccharide in symbiosis. Previous reports have shown that the overexpression of rkpZ from strain Rm41 allows for the symbiosis of exoY mutants of Rm1021 that are unable to produce the exopolysaccharide succinoglycan. Our results demonstrate that mutations in the rkp-1 cluster prevent this phenotypic suppression of exoY mutants, although mutations in kdsB2, smb20804, and smb20805 have no effect.

* Corresponding author. Mailing address: Great Lakes Bioenergy Research Center, 3552 Microbial Sciences, 1550 Linden Dr., University of Wisconsin-Madison, Madison, WI 53706. Phone: (608) 890-2547. Fax: (608) 890-2515. E-mail: dkeating{at}glbrc.wisc.edu

{triangledown} Published ahead of print on 4 September 2009.

{dagger} Supplemental material for this article may be found at http://jb.asm.org/.

Journal of Bacteriology, November 2009, p. 6988-7000, Vol. 191, No. 22
0021-9193/09/$08.00+0     doi:10.1128/JB.00466-09
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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