Structures of exopolysaccharides involved in receptor-mediated perception of Mesorhizobium loti by Lotus japonicus

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  • Artur Muszyński, Complex Carbohydrate Research Center, University of Georgia, United States; muszynski@ccrc.uga.edu.
  • ,
  • Christian Heiss, Complex Carbohydrate Research Center, University of Georgia, United States;
  • ,
  • Christian T Hjuler, Department of Medical Chemistry, University of Copenhagen, Denmark
  • ,
  • John T Sullivan, Department of Microbiology and Immunology, University of Otago, New Zealand;
  • ,
  • Simon J Kelly
  • Jens Stougaard
  • Parastoo Azadi, Complex Carbohydrate Research Center, University of Georgia, United States;
  • ,
  • Russell W Carlson, Complex Carbohydrate Research Center, University of Georgia, United States;
  • ,
  • Clive W Ronson, Department of Microbiology and Immunology, University of Otago, New Zealand;

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. 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.

OriginalsprogEngelsk
TidsskriftJournal of Biological Chemistry
Vol/bind291
Nummer40
Sider (fra-til)20946-20961
Antal sider16
ISSN0021-9258
DOI
StatusUdgivet - 8 aug. 2016

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