HOST-SPECIFICITY OF RHIZOBIUM-LEGUMINOSARUM IS DETERMINED BY THE HYDROPHOBICITY OF HIGHLY UNSATURATED PATTY ACYL MOIETIES OF THE NODULATIONFACTORS

The nodE genes of Rhizobium leguminosarum bvs, trifolii and viciae str ains are the major determinants of the host range of nodulation, Using mass spectrometry we have analyzed the structures of the major and mi nor lipooligosaccharides produced by two R, leguminosarum strains that differ only in the origin of the nod genes, The strain containing the nod genes of R. l. by. viciae produces lipo-chitin oligosaccharide (L CO) molecules that contain the common cis-vaccenyl (C18:1) or a highly unsaturated acyl (C18:4) group that is determined by nodE (Spaink et al, Nature 354:125-130, 1991), Here we show that, in addition, minor q uantities of analogous molecules that contain different common fatty a cyl groups such as stearyl (C18:0), palmityl (C16:0), and palmitoyl (C 16:1) groups are produced, The set of LCOs produced by the strain cont aining the R. l. by. trifolii nod genes is similar, but major differen ces are found in the length and unsaturation of the fatty acyl groups of various LCOs. The R. l, by. trifolii strain does not produce LCO mo lecules that contain the C18:4 fatty acyl group. Instead, a complex mi xture of other unsaturated fatty acyl groups containing either two, th ree, or four double bonds is produced, These novel fatty acyl groups a re more hydrophobic than the C18:4 fatty acyl group found in by. vicia e, as judged by highperformance liquid chromatography retention times, probably as a result of the longer carbon chain length, the lack of a cis double bond, or both, Another difference is that the relative amo unt of highly unsaturated fatty acid-containing LCOs produced by R. l. by. trifolii is much smaller An isogenic R. l. by. trifolii strain co ntaining a Tn5 insertion in the nodE gene only produces LCOs containin g the common fatty acyl groups and not the highly unsaturated types, T he results indicate that the difference in the host range of the R. l. bvs, viciae and trifolii is determined by the overall hydrophobicity of the highly unsaturated fatty acyl moieties of LCOs rather than by a specific structural feature.