IDENTIFICATION OF NODSUIJ GENES IN NOD LOCUS-1 OF AZORHIZOBIUM-CAULINODANS - EVIDENCE THAT NODS ENCODES A METHYLTRANSFERASE INVOLVED IN NODFACTOR MODIFICATION

The Azorhizobium caulinodans strain ORS571 nodulation genes nodSUIJ we re located downstream from nodABC. Complementation data and transcript ional analysis suggest that nodABCSUIJ form a single operon. Mutants w ith Tn5 insertions in the genes nodS, nodU, and nodJ were delayed in n odulation of Sesbania rostrata roots and stems. The NodS amino acid se quences of ORS571, Bradyrhizobium japonicum, and Rhizobium sp. strain NGR234, contain a consensus with similarity to S-adenosylmethionine (S AM)-utilizing methyltransferases. A naringenin-inducible nodS-dependen t protein of approximately 25 kDa could be cross-linked to radiolabell ed SAM. By applying L-[methyl-H-3]-methionine in vivo, Nod factors of ORS571, known to be N-methylated, could be labelled in wild type and n odU mutants but not in nodS mutants. Therefore, we propose that NodS i s a SAM-utilizing methyltransferase involved in Nod factor synthesis.