IN RHIZOBIUM-MELILOTI, THE OPERON ASSOCIATED WITH THE NOD BOX N5 COMPRISES NODL, NOEA AND NOEB, 3 HOST-RANGE GENES SPECIFICALLY REQUIRED FOR THE NODULATION OF PARTICULAR MEDICAGO SPECIES

In Rhizobium meliloti, the genes required for nodulation of legume hos ts are under the control of DNA regulatory sequences called nod boxes. In this paper, we have characterized three host-specific nodulation g enes, which form a flavonoid-inducible operon downstream of the nod bo x n5. The first gene of this operon is identical to the nodL gene iden tified by Baev and Kondorosi (1992) in R. meliloti strain AK631. The p roduct of the second gene, NoeA, presents some homology with a methyl transferase. nodL mutants synthesize Nod factors lacking the O-acetate substituent. In contrast, in strains carrying a mutation in either no eA or noeS, no modification in Nod-factor structure or production coul d be detected. On particular hosts, such as Medicago littoralis, mutan ts of the n5 operon showed a very weak nodule-forming ability, associa ted with a drastic decrease in the number of infection threads, while nodulation of Medicago truncatula or Melilotus alba was not affected. Thus, nodL, noeA and noeB are host-specific nodulation genes. By using a gain-of-function approach, we showed that the presence of nodL, and hence of O-acetylated Nod factors, is a major prerequisite for confer ing the ability to nodulate alfalfa upon the heterologous bacterium Rh izobium tropici.