Novel branched nod factor structure results from alpha-(1-->3) fucosyl transferase activity: the major lipo-chitin oligosaccharides from Mesorhizobium loti strain NZP2213 bear an alpha-(1-->3) fucosyl substituent on a nonterminal backbone residue.

Mesorhizobium loti has been described as a microsymbiont of plants of the genus Lotus. Lipo-chitin oligosaccharides (LCOs), or Nod factors, produced by several representative M. loti strains all have similar structures. Using fast-atom-bombardment tandem mass spectrometry and NMR spectroscopy, we have now examined the LCOs from the type strain NZP2213 and observed a much greater variety of structures than has been described for the strains of M.loti studied previously. Interestingly, we have identified as the major LCO a structure that bears a fucose residue alpha-1,3-linked to the GlcNAc residue proximal to the nonreducing terminal GlcNAc residue. This is the first time, to our knowledge, that substitution on an internal GlcNAc residue of the LCO backbone has been observed. This novel LCO structure suggests the presence of a novel fucosyltransferase activity in strain NZP2213. Since the presence of this extra structure does not have the effect of broadening the host range, we suggest that the modification of the LCOs with a fucose residue linked to a nonterminal GlcNAc residue might provide protection against degradation by a particular host plant enzyme (e.g., a chitinase) or alternatively represents adaptation to a particular host-specific receptor. The action of the alpha-(1-->3) fucosyltransferase seems to reduce significantly the activity of NodS, the methyltransferase involved in the addition of the N-methyl substituent to the nonreducing terminal GlcNAc residue. An additional novel LCO structure has been identified having only a GlcNAc2 backbone. This is to our knowledge the first description of such a minimal LCO structure.