SULFATION OF RHIZOBIUM SP NGR234 NOD-FACTORS IS DEPENDENT ON NOEE, A NEW HOST-SPECIFICITY GENE

Rhizobia secrete specific lipo-chitooligosaccharide signals (LCOs) cal led Nod factors that are required for infection and nodulation of legu mes. In Rhizobium sp. NGR234, the reducing N-acetyl-D-glucosamine of L COs is substituted at C-6 with 2-O-methyl-L-fucose which can be acetyl ated or sulphated. We identified a flavonoid-inducible locus on the sy mbiotic plasmid pNGR234a that contains a new nodulation gene, noeE, wh ich is required for the sulphation of NGR234 Nod factors (NodNGR). noe E was identified by conjugation into the closely related Rhizobium fre dii strain USDA257, which produces fucosylated but non-sulphated Nod f actors (NodUSDA). R. fredii transconjugants producing sulphated LCOs a cquire the capacity to nodulate Calopogonium caeruleum. Furthermore, m utation of noeE (NGR Delta noeE) abolishes the production of sulphated LCOs and prevents nodulation of Pachyrhizus tuberosus. The sulphotran sferase activity linked to NoeE is specific for fucose. In contrast, t he sulphotransferase NodH of Rhizobium meliloti seems to be less speci fic than NoeE, because its introduction into NGR Delta noeE leads to t he production of a mixture of LCOs that are sulphated on C-6 Of the re ducing terminus and sulphated on the P-O-methylfucose residue. Togethe r, these findings show that noeE is a host-specificity gene which prob ably encodes a fucose-specific sulphotransferase.