THE COMMON NODABC GENES OF RHIZOBIUM-MELILOTI ARE HOST-RANGE DETERMINANTS

Symbiotic bacteria of the genus Rhizobium synthesize lipo-chitooligosa ccharides, called Nod factors (NFs), which act as morphogenic signal m olecules on legume hosts. The common nodABC genes, present in all Rhiz obium species, are required for the synthesis of the core structure of NFs. NodC is an N-acetylglucosaminyltransferase, and NodB is a chitoo ligosaccharide deacetylase; NodA is involved in N-acylation of the ami nosugar backbone. Specific nod genes are involved in diverse NF substi tutions that confer plant specificity. We transferred to R. tropici, a broad host-range tropical symbiont, the ability to nodulate alfalfa, by introducing nod genes of R. meliloti. In addition to the specific n odL and nodFE genes, the common nodABC genes of R. meliloti were requi red for infection and nodulation of alfalfa. Purified NFs of the R. tr opici hybrid strain, which contained chitin tetramers and were partly N-acylated with unsaturated C16 fatty acids, were able to elicit nodul e formation on alfalfa. Inactivation of the R. meliloti nodABC genes s uppressed the ability of the NFs to nodulate alfalfa. Studies of NFs f rom nodA, nodB, nodC, and nodI mutants indicate that (i) NodA of R. me liloti, in contrast to NodA of R. tropici, is able to transfer unsatur ated C16 fatty acids onto the chitin backbone and (ii) NodC of R. meli loti specifies the synthesis of chitin tetramers. These results show t hat allelic variation of the common nodABC genes is a genetic mechanis m that plays an important role in signaling variation and in the contr ol of host range.