Mesorhizobium sp. strain N33 (Oxytropis arctobia), a rhizobial strain isolated in arctic Canada, is able to fix nitrogen at very low temperatures in association with a few arctic legume species belonging to the genera Astragalus, Onobrychis, and Oxytropis. Using mass spectrometry and nuclear magnetic resonance spectroscopy, we have determined the structure of N33 Nod factors, which are major determinants of nodulation. They are pentameric lipochito-oligosaccharides 6-O sulfated at the reducing end and exhibit other original substitutions: 6-O acetylation of the glucosamine residue next to the nonreducing terminal glucosamine and N acylation of the nonreducing terminal glucosamine by methyl-branched acyl chains of the iso series, some of which are ␣,␤ unsaturated. These unusual substitutions may contribute to the peculiar host range of N33. Analysis of N33 whole-cell fatty acids indicated that synthesis of the methyl-branched fatty acids depended on the induction of bacteria by plant flavonoids, suggesting a specific role for these fatty acids in the signaling process between the plant and the bacteria. Synthesis of the methyl-branched ␣,␤-unsaturated fatty acids required a functional nodE gene.Rhizobia are soil bacteria, now classified in several genera (e.g., Rhizobium, Sinorhizobium, Mesorhizobium, Bradyrhizobium, Azorhizobium), which form a symbiotic association with legume plants. The interaction between bacteria and plants results in the formation of nodules on the host plant roots, in which rhizobia fix atmospheric nitrogen. The association between rhizobia and legumes is specific: each rhizobial strain has a defined host range. For example, Mesorhizobium sp. strain N33, isolated from Oxytropis arctobia, also nodulates Astragalus alpinus and Onobrychis viciifolia (19,26). In contrast, Sinorhizobium meliloti efficiently nodulates alfalfa (Medicago sativa) as well as Melilotus and Trigonella species. Mesorhizobium sp. strain N33 was isolated in the Canadian high arctic and is able to grow and fix nitrogen at temperatures as low as 5°C. In addition, it was shown that arctic rhizobia promoted better growth of O. viciifolia at low temperatures than did temperate strains (27). Thus, it might be valuable to extend the host range of arctic rhizobia to agronomically important legumes, in order to improve nitrogen fixation by these plants at low temperatures. It is therefore important to understand the molecular mechanisms controlling the nodulation specificity of arctic rhizobia.Earlier work has shown that nodulation and host specificity in rhizobium-legume symbiosis are determined by signal exchanges between the bacteria and the host plant (21). Flavonoids excreted by the plant roots induce the expression of rhizobial nodulation (nod) genes. Most of these genes are involved in the biosynthesis and secretion of bacterial signals, the Nod factors, that can specifically induce symbiotic responses of the host plants. These responses include root hair deformation, division of root cortical cells and, in some instances,...