NOLO AND NOEI (HSNIII) OF RHIZOBIUM SP. NGR234 ARE INVOLVED IN 3-O-CARBAMOYLATION AND 2-O-METHYLATION OF NOD FACTORS

Loci unique to specific rhizobia direct the adjunction of special grou ps to the core lipo-oligosaccharide Nod factors, Host-Specificity of n odulation (Hsn) genes are thus essential for interaction with certain legumes. Rhizobium sp, NGR234, which can nodulate >110 genera of legum es, possesses three hsn loci and secretes a large family of Nod factor s carrying specific substituents. Among them are 3-O (or 4-O)- and 8-O -carbamoyl groups, an N-methyl group, and a 2-O-methylfucose residue w hich 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 n odABCIJnoLOnoeI, Complementation and mutation analyses show that the d isruption of any one of nodIJ, nolO, or noeI has no effect on nodulati on, Conjugation of nolO into Rhizobium fredii extends the host range o f the recipient to the non-hosts Calopogonium caeruleum and Lablab pur pureus, however. Chemical analyses of the Nod factors produced by the NodI, NolO, and NoeI mutants show that the nolO and noel gene products are required for 3 (or 4)-O-carbamoylation of the nonreducing terminu s and for 2-O-methylation of the fucosyl group, respectively. Confirma tion that NolO is a carbamoyltransferase was obtained from analysis of the Nod factors produced by R. fredii containing nolO; all are carbam oylated at O-3 (or O-4) on the nonreducing terminus. Since mutation of both nolO and nodU fails to completely abolish production of monocarb amoylated NodNGR factors, it is clear that a third carbamoyltransferas e 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.