Rhizobia synthesize mono-N-acylated chitooligosaccharide signals, called Nod factors, that are required for the specific infection and nodulation of their legume hosts. The biosynthesis of Nod factors is under the control of nodulation (nod) genes, including the nodABC genes present in all rhizobial species. The N-acyl substitution can vary between species and can play a role in host specificity. In Rhizobium meliloti, an alfalfa symbiont, the acyl chain is a C16 unsaturated or a (omega-1) hydroxylated fatty acid, whereas in Rhizobium tropici, a bean symbiont, it is vaccenic acid (C18:1). We constructed R. meliloti derivatives having a non-polar deletion of nodA, and carrying a plasmid with either the R. meliloti or the R. tropici nodA gene. The strain with the R. tropici nodA gene produced Nod factors acylated by vaccenic acid, instead of the C16 unsaturated or hydroxylated fatty acids characteristic of R. meliloti Nod factors, and infected and nodulated alfalfa with a significant delay. These results show that NodA proteins of R. meliloti and R. tropici specify the N-acylation of Nod factors by different fatty acids, and that allelic variation of the common nodA gene can contribute to the determination of host range.
We have isolated and characterized the extracellular Nod factors of Rhizobium fredii USDA257, a nitrogen-fixing symbiont of soybean [Glycine max (L.) Merr.] and several other legume species. These signals are produced upon exposure to the isoflavone genistein and consist of a series of substituted, beta 1,4-linked tri-, tetra-, and pentamers of N-acetylglucosamine. N-Vaccenic acid replaces acetate on the nonreducing residue, and the reducing residue contains alpha-linked 2-O-methylfucose on carbon 6. Small amounts of a fucose-containing tetramer also were present. The Nod factors elicit root-hair deformations on soybean and two other plants at concentrations ranging from 10(-6) to 10(-12) M.
Structure of the Mesorhizobium huakuii and Rhizobium galegae Nod factors: a cluster of phylogenetically related legumes are nodulated by rhizobia producing Nod factors with a,b-unsaturated N-acyl substitutions
SummaryRhizobia are symbiotic bacteria that synthesize lipochitooligosaccharide Nod factors (NFs), which act as signal molecules in the nodulation of speci®c legume hosts. Based on the structure of their N-acyl chain, NFs can be classi®ed into two categories: (i) those that are acylated with fatty acids from the general lipid metabolism; and (ii) those ( aU-NFs) that are acylated by speci®c a,b-unsaturated fatty acids (containing carbonyl-conjugated unsaturation(s)). Previous work has described how rhizobia that nodulate legumes of the Trifolieae and Vicieae tribes produce aU-NFs. Here, we have studied the structure of NFs from two rhizobial species that nodulate important genera of the Galegeae tribe, related to Trifolieae and Vicieae. Three strains of Mesorhizobium huakuii, symbionts of Astragalus sinicus, produced as major NFs, pentameric lipochitooligosaccharides O-sulphated and partially N-glycolylated at the reducing end and N-acylated, at the non-reducing end, by a C18:4 fatty acid. Two strains of Rhizobium galegae, symbionts of Galega sp., produced as major NFs, tetrameric O-carbamoylated NFs that could be O-acetylated on the glucosamine residue next to the non-reducing terminal glucosamine and were N-acylated by C18 and C20 a,b-unsaturated fatty acids. These results suggest that legumes nodulated by rhizobia synthesizing aUNFs constitute a phylogenetic cluster in the Galegoid phylum.
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