Structure of the Mesorhizobium huakuii and Rhizobium galegae Nod factors: a cluster of phylogenetically related legumes are nodulated by rhizobia producing Nod factors with alpha,beta-unsaturated N-acyl substitutions

Rhizobia are symbiotic bacteria that synthesize lipochitooligosaccharide No d factors (NFs), which act as signal molecules in the nodulation of specifi c legume hosts. Based on the structure of their N-acyl chain, NFs can be cl assified into two categories: (i) those that are acylated with fatty acids from the general lipid metabolism; and (ii) those (=alpha U-NFs) that are a cylated by specific alpha,beta-unsaturated fatty acids (containing carbonyl -conjugated unsaturation(s)). Previous work has described how rhizobia that nodulate legumes of the Trifolieae and Vicieae tribes produce alpha U-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 Astragalos s inicos, produced as major NFs, pentameric lipochitooligosaccharides O-sulph ated and partially N-glycolylated at the reducing end and N-acylated, at th e non-reducing end, by a C18:4 fatty acid, Two strains of Rhizobium galegae , symbionts of Galega sp., produced as major NFs, tetrameric O-carbamoylate d 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 alpha,bet a-unsaturated fatty acids. These results suggest that legumes nodulated by rhizobia synthesizing alpha U-NFs constitute a phylogenetic cluster in the Galegoid phylum.