ABSTRACT
Following
the initial discovery of two legume-nodulating Burkholderia
strains (L. Moulin, A. Munive, B. Dreyfus, and C.
Boivin-Masson, Nature 411:948-950, 2001), we identified as
nitrogen-fixing legume symbionts at least 50 different strains of
Burkholderia caribensis and Ralstonia
taiwanensis, all belonging to the β-subclass of
proteobacteria, thus extending the phylogenetic diversity of the
rhizobia. R. taiwanensis was found to represent
93% of the Mimosa isolates in Taiwan, indicating thatβ
-proteobacteria can be the specific symbionts of a legume. The
nod genes of rhizobial β-proteobacteria
(β-rhizobia) are very similar to those of rhizobia from theα
-subclass (α-rhizobia), strongly supporting the
hypothesis of the unique origin of common nod genes. Theβ
-rhizobial nod genes are located on a 0.5-Mb plasmid,
together with the nifH gene, in R.
taiwanensis and Burkholderia phymatum.
Phylogenetic analysis of available nodA gene sequences
clustered β-rhizobial sequences in two nodA lineages
intertwined with α-rhizobial sequences. On the other hand, theβ
-rhizobia were grouped with free-living nitrogen-fixingβ
-proteobacteria on the basis of the nifH phylogenetic
tree. These findings suggest that β-rhizobia evolved from
diazotrophs through multiple lateral nod gene
transfers.
Conservation of symbiotic nitrogen fixation gene sequences in Rhizobium japonicum and Bradyrhizobium japonicum.