INVOLVEMENT OF NODS IN N-METHYLATION AND NODU IN 6-O-CARBAMOYLATION OF RHIZOBIUM SP NGR234 NOD FACTORS

Although Rhizobium sp, NGR234 and Rhizobium fredii USDA257 share many traits, dysfunctional nodSU genes in the latter prohibit nodulation of Leucaena species, Accordingly, we used R. fredii transconjugants harb oring the nodS and nodU genes of NGR234 to study their role in the str uctural modification of the Lipo-oligosaccharide Nod factors, Differen ces between the Nod factors mainly concern the length of the oligomer (three to five glucosamine residues in USDA257 and five residues only in NGR234) and the presence of additional substituents in NGR234 (N-li nked methyl, one or two carbamoyl groups on the non-reducing moiety, a cetyl or sulfate groups on the fucose), R. fredii(nodS) transconjugant s produce chitopentamer Nod factors with a N-linked methyl group on th e glucosaminyl terminus. Introduction of nodU into USDA257 results in the formation of 6-O-carbamoylated factors. Co-transfer of nodSU direc ts N-methylation, mono-6-O-carbamoylation, and production of pentameri c Nod factors. Mutation of nodU in NGR234 suppresses the formation of bis-carbamoylated species. Insertional mutagenesis of nodSU drasticall y decreases Nod factor production, but with the exception of sulfated factors (which are partially N-methylated and mono-carbamoylated), the y are identical to those of the wild-type strain. Thus, Nod factor lev els, their degree of oligomerization, and N-methylation are linked to the activity encoded by nodS.