Rhizobium Nodulation Factors: Variations on a Theme

Abstract

Nodulation (nod) genes of Rhizobium, which control infection, nodulation and host specificity, determine the production of extracellular lipo-oligosaccharidic Nod factors. The Nod factors of various rhizobia, including the narrow host-range temperate R. meliloti and the broad host-range tropical Rhizobium sp. NGR234 belong to the same chemical family: they are mono-N-acylated and substituted chitin oligomers. The common nodC gene codes for a protein which has homology with chitin synthases and may be involved in the synthesis of the chitin backbone. The species-specific nod genes determine the decoration of these core molecules by encoding substitutions with specific groups at particular sites. For example in R. meliloti (i) the nodH and nodPQ genes are involved in the O-sulfation of the lipooligosaccharides on the carbon 6 of the reducing glucosamine; (ii) the nodL gene is required for the O-acetylation of the carbon 6 of the non-reducing terminal glucosamine; (iii) the nodFE genes specify the synthesis of the specific C16:2 N-acyl chain. Rhizobium sp. NGR234 secretes a large family of Nod factors carrying a variety of substituents. The non-reducing end is N-acylated with vaccenic or palmitic acids, is N-methylated, and carries varying numbers of carbamoyl groups. The reducing glucosamine residue is substituted on position 6 with 2-O-methyl-L-fucose which may be acetylated, or sulfated, or non-substituted. The O-acetylation and the O-sulfation are mutually exclusive: thus this broad host range strain secretes a variety of charged and uncharged Nod factors. We also present data which strongly suggest that the host range nod genes mediate host specificity by determining the type of substitutions decorating the lipo-oligosaccharidic Nod factors.