The biosynthesis of rhizobial lipo-oligosaccharide nodulation signal molecules

While a great deal has been learned concerning the biosynthesis of Nod factors, there is much that remains to be determined. The functions of many Nod proteins involved in adding the host-specific modifications to the Nod factors remain to be unequivocally identified. Some of the genes required for these modifications have not yet been isolated, e.g., those involved in carbamylation, or addition of D-Ara. Additionally the cellular location of most of the Nod proteins and, concomitantly, the modifications they determine are not known. The actual in vivo substrates for the NodABC proteins have not been identified, and the enzyme activities of purified NodA and NodC have not been demonstrated. The synthesis and export of the Nod factors most probably involves some type of carrier/anchor which remains unidentified. Analysis of GlcNAc metabolites from various mutants, e.g., nodA-, nodB-, or nodC- mutants, should facilitate the identification of the in vivo substrates involved in the synthesis of the "common" Nod factor and, thereby, lead to a greater understanding of Nod factor biosynthesis and transport. Finally, comparison of Nod factor biosynthesis to other examples of polysaccharide or glycolipid biosynthetic pathways suggest that several key enzymes remain to be identified. It is hoped that this discussion will be helpful in designing strategies for the detection and isolation of such novel enzymes.