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Glycobiology Advance Access published online on September 6, 2006
Glycobiology, doi:10.1093/glycob/cwl042
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© The Author 2006. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org
Received April 12, 2006
Revised August 28, 2006
Accepted August 31, 2006

Article

Overproduction and increased molecular weight account for the symbiotic activity of the rkpZ-modified K polysaccharide from Sinorhizobium meliloti Rm1021

L.A. Sharypova 1 *, G. Chataigné 2, N. Fraysse 1, A. Becker 1, and V. Poinsot 2

1 Department of Genetics, Biology VI, Bielefeld University, Postfach 100131, 33501 Bielefeld, Germany
2 Spectrométrie de masse, Fluorescence et Chimie Bioanalytique Laboratoire des IMRCP 118, route de Narbonne, F-31062 Toulouse-Cedex, France

* To whom correspondence should be addressed.
L.A. Sharypova, E-mail: larissa{at}Genetik.Uni-Bielefeled.de


  Abstract

K polysaccharides of Sinorhizobium meliloti strains are strain-specific surface polysaccharides analogous to the group II K antigens of Escherichia coli. The KR5 antigen of strain AK631 is a highly polymerized disaccharide of pseudaminic and glucuronic acids. During invasion of host plants, this K antigen is able to replace the structurally different exopolysaccharide succinoglycan (EPS I) and promotes the formation of a nitrogen-fixing (Fix+) symbiosis. The K polysaccharide of strain Rm1021 is a homopolymer of 3-deoxy-D-manno-2 octulosonic acid (Kdo). The Kdo polysaccharide is covalently linked to the lipid anchor, has a low molecular weight, and is symbiotically inactive. Upon introduction of the Rm41-specific rkpZ gene into strain Rm1021, a modified K polysaccharide is expressed that is able to substitute EPS I during symbiosis with the host plant. To better understand the nature of modification conferred by rkpZ, we performed a structural analysis of the K polysaccharide using NMR, ESI-MS and GC-MS. The modified K polysaccharide retained primary polyKdo structure, but its degree of polymerization and level of production were increased significantly. In contrast to the wild-type polyKdo, only a part of polyKdo was lipidated. Shorter polysaccharide chains were lipid-free, while longer polysaccharide chains were lipidated. S. meliloti Rm1021 was found to carry two paralogs of rkpZ. Both genes are involved in polyKdo production, but they only show partial functional activity as compared to the rkpZ of Rm41.

Keywords: electrospray-ionization mass spectrometry/host invasion/lipid anchor/rkpZ/surface polysaccharide.
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