title: Structures of Exopolysaccharides Involved in Receptor-mediated Perception of Mesorhizobium loti by Lotus japonicus.
abstract: In the symbiosis formed between Mesorhizobium loti strain R7A and Lotus japonicus Gifu, rhizobial exopolysaccharide (EPS) plays an important role in infection thread formation. Mutants of strain R7A affected in early exopolysaccharide biosynthetic steps form nitrogen-fixing nodules on L. japonicus Gifu after a delay, whereas mutants affected in mid or late biosynthetic steps induce uninfected nodule primordia. Recently, it was shown that a plant receptor-like kinase, EPR3, binds low molecular mass exopolysaccharide from strain R7A to regulate bacterial passage through the plant's epidermal cell layer (Kawaharada, Y., Kelly, S., Nielsen, M. W., Hjuler, C. T., Gysel, K., Muszyński, A., Carlson, R. W., Thygesen, M. B., Sandal, N., Asmussen, M. H., Vinther, M., Andersen, S. U., Krusell, L., Thirup, S., Jensen, K. J., et al. (2015) Nature 523, 308-312). In this work, we define the structure of both high and low molecular mass exopolysaccharide from R7A. The low molecular mass exopolysaccharide produced by R7A is a monomer unit of the acetylated octasaccharide with the structure (2,3/3-OAc)β-d-RibfA-(1→4)-α-d-GlcpA-(1→4)-β-d-Glcp-(1→6)-(3OAc)β-d-Glcp-(1→6)-*[(2OAc)β-d-Glcp-(1→4)-(2/3OAc)β-d-Glcp-(1→4)-β-d-Glcp-(1→3)-β-d-Galp]. We propose it is a biosynthetic constituent of high molecular mass EPS polymer. Every new repeating unit is attached via its reducing-end β-d-Galp to C-4 of the fourth glucose (asterisked above) of the octasaccharide, forming a branch. The O-acetylation occurs on the four glycosyl residues in a non-stoichiometric ratio, and each octasaccharide subunit is on average substituted with three O-acetyl groups. The availability of these structures will facilitate studies of EPR3 receptor binding of symbiotically compatible and incompatible EPS and the positive or negative consequences on infection by the M. loti exo mutants synthesizing such EPS variants.
authors: From the Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 30602, muszynski@ccrc.uga.edu.
authors: Artur
authors: http://orcid.org/0000-0003-3497-9977
authors: A
authors: Muszyński
authors: From the Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 30602.
authors: Christian
authors: C
authors: Heiss
authors: the Department of Chemistry, University of Copenhagen, 1871 Frederiksberg C, Denmark.
authors: Christian T
authors: http://orcid.org/0000-0001-6055-6391
authors: CT
authors: Hjuler
authors: the Department of Microbiology and Immunology, University of Otago, Dunedin 9016, New Zealand.
authors: John T
authors: JT
authors: Sullivan
authors: the Department of Microbiology and Immunology, University of Otago, Dunedin 9016, New Zealand.
authors: Simon J
authors: SJ
authors: Kelly
authors: the Department of Chemistry, University of Copenhagen, 1871 Frederiksberg C, Denmark.
authors: Mikkel B
authors: http://orcid.org/0000-0002-0158-2802
authors: MB
authors: Thygesen
authors: the Department of Molecular Biology and Genetics, Aarhus University, 8000 Aarhus C, Denmark, and.
authors: Jens
authors: http://orcid.org/0000-0002-9312-2685
authors: J
authors: Stougaard
authors: From the Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 30602.
authors: Parastoo
authors: http://orcid.org/0000-0002-6166-9432
authors: P
authors: Azadi
authors: From the Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 30602.
authors: Russell W
authors: RW
authors: Carlson
authors: the Department of Microbiology and Immunology, University of Otago, Dunedin 9016, New Zealand, Clive.Ronson@otago.ac.nz.
authors: Clive W
authors: http://orcid.org/0000-0002-2217-9676
authors: CW
authors: Ronson
mesh_terms: D002236:Carbohydrate Conformation; D000070116:Lotus / Q000235:genetics / Q000378:metabolism* / Q000382:microbiology; D059128:Mesorhizobium / Q000235:genetics / Q000378:metabolism*; D009154:Mutation; D019441:Plant Epidermis / Q000235:genetics / Q000378:metabolism* / Q000382:microbiology; D011135:Polysaccharides, Bacterial / Q000235:genetics / Q000378:metabolism*; D013559:Symbiosis / Q000502:physiology*
chemical_list: D011135:Polysaccharides, Bacterial
keywords: Mesorhizobium loti; acetylation; carbohydrate structure; exopolysaccharide; octasaccharide; oligosaccharide; polysaccharide; rhizobia; riburonic acid; symbiosis