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
M. Ardourel et al., 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, Molecular microbiology, 17(4), 1995, pp. 687-699
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.