nolO and noeI (HsnIII) of Rhizobium sp. NGR234 are involved in 3-O-carbamoylation and 2-O-methylation of Nod factors.

ABSTRACT

Loci unique to specific rhizobia direct the adjunction of special groups to the core lipo-oligosaccharide Nod factors. Host-specificity of nodulation (Hsn) genes are thus essential for interaction with certain legumes. Rhizobium sp. NGR234, which can nodulate >110 genera of legumes, possesses three hsn loci and secretes a large family of Nod factors carrying specific substituents. Among them are 3-O (or 4-O)- and 6-O-carbamoyl groups, an N-methyl group, and a 2-O-methylfucose residue which may bear either 3-O-sulfate or 4-O (and 3-O)-acetyl substituents. The hsnIII locus comprises a nod box promoter followed by the genes nodABCIJnolOnoeI. Complementation and mutation analyses show that the disruption of any one of nodIJ, nolO, or noeI has no effect on nodulation. Conjugation of nolO into Rhizobium fredii extends the host range of the recipient to the non-hosts Calopogonium caeruleum and Lablab purpureus, however. Chemical analyses of the Nod factors produced by the NodI, NolO, and NoeI mutants show that the nolO and noeI gene products are required for 3 (or 4)-O-carbamoylation of the nonreducing terminus and for 2-O-methylation of the fucosyl group, respectively. Confirmation that NolO is a carbamoyltransferase was obtained from analysis of the Nod factors produced by R. fredii containing nolO; all are carbamoylated at O-3 (or O-4) on the nonreducing terminus. Since mutation of both nolO and nodU fails to completely abolish production of monocarbamoylated NodNGR factors, it is clear that a third carbamoyltransferase must exist. Nevertheless, the specificities of the two known enzymes are clearly different. NodU is only able to transfer carbamate to O-6 while NolO is specific for O-3 (or O-4) of NodNGR factors.