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J. Biol. Chem., Vol. 283, Issue 23, 16037-16050, June 6, 2008

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Structural Characterization of the Primary O-antigenic Polysaccharide of the Rhizobium leguminosarum 3841 Lipopolysaccharide and Identification of a New 3-Acetimidoylamino-3-deoxyhexuronic Acid Glycosyl Component

A UNIQUE O-METHYLATED GLYCAN OF UNIFORM SIZE, CONTAINING 6-DEOXY-3-O-METHYL-D-TALOSE, N-ACETYLQUINOVOSAMINE, AND RHIZOAMINURONIC ACID (3-ACETIMIDOYLAMINO-3-DEOXY-D-GLUCO-HEXURONIC ACID)^*

L. Scott Forsberg and Russell W. Carlson¹

From the Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 30605

Rhizobium are Gram-negative bacteria that survive intracellularly, within host membrane-derived plant cell compartments called symbiosomes. Within the symbiosomes the bacteria differentiate to bacteroids, the active form that carries out nitrogen fixation. The progression from free-living bacteria to bacteroid is characterized by physiological and morphological changes at the bacterial surface, a phase shift with an altered array of cell surface glycoconjugates. Lipopolysaccharides undergo structural changes upon differentiation from the free living to the bacteroid (intracellular) form. The array of carbohydrate structures carried on lipopolysaccharides confer resistance to plant defense mechanisms and may serve as signals that trigger the plant to allow the infection to proceed. We have determined the structure of the major O-polysaccharide (OPS) isolated from free living Rhizobium leguminosarum 3841, a symbiont of Pisum sativum, using chemical methods, mass spectrometry, and NMR spectroscopy analysis. The OPS is composed of several unusual glycosyl residues, including 6-deoxy-3-O-methyl-D-talose and 2-acetamido-2deoxy-L-quinovosamine. In addition, a new glycosyl residue, 3-acetimidoylamino-3-deoxy-D-gluco-hexuronic acid was identified and characterized, a novel hexosaminuronic acid that does not have an amino group at the 2-position. The OPS is composed of three to four tetrasaccharide repeating units of 4)-β-DGlcp3NAmA-(14)-[2-O-Ac-3-O-Me--D-6dTalp-(13)]--L-Fucp-(13)--L-QuipNAc-(1. The unique 3-amino hexuronate residue, rhizoaminuronic acid, is an attractive candidate for selective inhibition of OPS synthesis.

Received for publication, November 26, 2007 , and in revised form, April 1, 2008.

^* This work was supported, in whole or in part, by National Institutes of Health Grant GM39583 (to R. W. C.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Experimental Procedures, additional references, Table 1, and Figs. S1-S12.

¹ To whom correspondence should be addressed. Tel.: 706-542-4439; Fax: 706-542-4412; E-mail: rcarlson{at}ccrc.uga.edu.

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