We determined the structures of Nod factors produced by six different Bradyrhizobium sp. strains nodulating the legume tree Acacia albida (syn. Faidherbia albida). Compounds from all strains were found to be similar, i.e., O-carbamoylated and substituted by an often sulfated methyl fucose and different from compounds produced by Rhizobium-Mesorhizobium-Sinorhizobium strains nodulating other species of the Acaciae tribe.Lipo-chitooligosaccharide Nod factors (NFs) synthesized by rhizobia act as signal molecules in the nodulation of specific legume hosts (23). NFs generally consist of four or five glucosamine residues that are N acylated at the nonreducing end and carry other substitutions on various glucosamine residues (7). Each rhizobial species (or biovar) has a defined host range and produces a set of NFs with specific structural features involved in host-range determination. Acacia species can be classified in three groups according to their ability to be nodulated in the field by fast-growing rhizobia of the RhizobiumSinorhizobium-Mesorhizobium branch (Acacia senegal, Acacia raddiana, and Acacia cyanophylla) (5,6,13,17), by Bradyrhizobium (Acacia albida, Acacia mangium, and Acacia auriculiformis) (10, 12) or by both types of rhizobia (Acacia seyal) (8). NFs of diverse Acacia nodulating fast-growing rhizobia, Sinorhizobium terangae bv. acaciae, Mesorhizobium plurifarium, Rhizobium sp. GRH2, and Rhizobium tropici, have been characterized and shown to be structurally very close (11,14,15,19). In particular, these molecules are not substituted by a glycosyl group, but they are mainly sulfated at the reducing end. NFs from Acacia bradyrhizobia have not yet been identified. However, fucosylated NF production seems to be a common feature of Bradyrhizobium (3, 22), thus raising the question as to whether rhizobia of the Rhizobium branch and Bradyrhizobium have evolved a similar or a different strategy, i.e., have developed similar or different NFs to nodulate Acacia species. Over the course of our studies on NF diversity in relation to host legume and bacterial taxonomy, we therefore examined structures produced by six genetically different Bradyrhizobium strains isolated from the leguminous tree A. albida. A. albida, recently reclassified as Faidherbia albida within the Acaciae tribe (18), is highly valuable in agroforestry for its soil improvement potential and as a source of wood and aerial forage (10).Strains and NF production and purification. A collection of A. albida nodulating strains was isolated in Sahelian and Sudano-Guinean areas (10) and was taxonomically characterized using both phenotypic (sodium dodecyl sulfate-polyacrylamide gel electrophoresis [SDS-PAGE] of whole protein extracts) and genotypic approaches. The isolates were shown to mainly belong to six protein electrophoretic clusters (clusters 1, 3, 4, 6, 7, and 8) belonging to the Bradyrhizobium rRNA branch (9).To evaluate NF diversity among A. albida bradyrhizobia and further select strains producing different molecules, 48 strains from the six...