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. Lipochitin oligosaccharides (LCOs), or Nod factors, p roduced by several representative M. loti strains all have similar str uctures. Using fast-atom-bombardment tandem mass spectrometry and NMR spectroscopy, we have now examined the LCOs from the type strain NZP22 13 and observed a much greater variety of structures than has been des cribed for the strains of M. loti studied previously. Interestingly, w e have identified as the major LCO a structure that bears a fucose res idue alpha-1,3-linked to the GlcNAc residue proximal to the nonreducin g terminal GlcNAc residue. This is the first time, to our knowledge, t hat substitution on an internal GlcNAc residue of the LCO backbone has been observed. This novel LCO structure suggests the presence of a no vel fucosyltransferase activity in strain NZP2213, Since the presence of this extra structure does not have the effect of broadening the hos t range, we suggest that the modification of the LCOs with a fucose re sidue linked to a nonterminal GlcNAc residue might provide protection against degradation by a particular host plant enzyme (e.g., a chitina se) or alternatively represents adaptation to a particular host-specif ic receptor. The action of the alpha-(1-->3) fucosyltransferase seems to reduce significantly the activity of NodS, the methyltransferase in volved in the addition of the N-methyl substituent to the nonreducing terminal GlcNAc residue. An additional novel LCO structure has been id entified having only a GlcNAc(2) backbone. This is to our knowledge th e first description of such a minimal LCO structure.