Résumé
Symbiotic nitrogen fixing bacteria—known as rhizobia—harbour a set of nodulation (
nod) genes that control the synthesis of modified lipo-chitooligosaccharides, called Nod factors that are required for legume nodulation. The
nodA gene, which is essential for symbiosis, is responsible for the attachment of the fatty acid group to the oligosaccharide backbone. The
nodZ,
nolL, and
noeI genes are involved in specific modifications of Nod factors common to bradyrhizobia, i.e., the transfer of a fucosyl group on the Nod factor core, fucose acetylation and fucose methylation, respectively. PCR amplification, sequencing and phylogenetic analysis of
nodA gene sequences from a collection of diverse
Bradyrhizobium strains revealed the monophyletic character with the possible exception of photosynthetic
Bradyrhizobium, despite high sequence diversity. The distribution of the
nodZ,
nolL, and
noeI genes in the studied strains, as assessed by gene amplification, hybridization or sequencing, was found to correlate with the
nodA tree topology. Moreover, the
nodA,
nodZ, and
noeI phylogenies were largely congruent, but did not closely follow the taxonomy of the strains shown by the housekeeping 16S rRNA and
dnaK genes. Additionally, the distribution of
nodZ,
noeI, and
nolL genes suggested that their presence may be related to the requirements of their legume hosts. These data indicated that the spread and maintenance of nodulation genes within the
Bradyrhizobium genus occurred through vertical transmission, although lateral gene transfer also played a significant role.