Unusual methyl-branched alpha,beta-unsaturated acyl chain substitutions inthe Nod factors of an arctic rhizobium, Mesorhizobium sp strain N33 (Oxytropis arctobia)

Mesorhizobium sp. strain N33 (Oxytropis arctobia), a rhizobial strain isola ted in arctic Canada, is able to fix nitrogen at very low temperatures in a ssociation with a few arctic legume species belonging to the genera Astraga lus, Onobrychis, and Oxytropis. Using mass spectrometry and nuclear magneti c resonance spectroscopy, we have determined the structure of N33 Nod facto rs, which are major determinants of nodulation. They are pentameric lipochi to-oligosaccharides 6-O sulfated at the reducing end and exhibit other orig inal substitutions: 6-O acetylation of the glucosamine residue next to the nonreducing terminal glucosamine and N acylation of the nonreducing termina l glucosamine by methyl-branched acyl chains of the iso series, some of whi ch are alpha,beta unsaturated. These unusual substitutions may contribute t o the peculiar host range of N33. Analysis of N33 whole-cell fatty acids in dicated that synthesis of the methyl-branched fatty acids depended on the i nduction of bacteria by plant flavonoids, suggesting a specific role for th ese fatty acids in the signaling process between the plant and the bacteria . Synthesis of the methyl-branched alpha,beta -unsatnrated fatty acids requ ired a functional nodE gene.