Classification of rhizobia based on nodC and nifH gene analysis reveals a close phylogenetic relationship among Phaseolus vulgaris symbionts

The nodC and nifH genes were characterized in a collection of 83 rhizobial strains which represented 23 recognized species distributed in the genera R hizobium, Sinorhizobium, Mesorhizobium and Bradyrhizobium, as well as uncla ssified rhizobia from various host legumes. Conserved primers were designed from available nucleotide sequences and were able to amplify nodC and nifH fragments of about 930 bp and 780 bp, respectively, from most of the strai ns investigated. RFLP analysis of the PCR products resulted in a classifica tion of these rhizobia which was in general well-correlated with their know n host range and independent of their taxonomic status. The nodC and nifH f ragments were sequenced for representative strains belonging to different g enera and species, most of which originated from Phaseolus vulgaris nodules . Phylogenetic trees were constructed and revealed close relationships amon g symbiotic genes of the Phaseolus symbionts, irrespective of their 16S-rDN A-based classification. The nodC and nifH phylogenies were generally simila r, but cases of incongruence were detected, suggesting that genetic rearran gements have occurred in the course of evolution. The results support the v iew that lateral genetic transfer across rhizobial species and, in some ins tances, across Rhizobium and Sinorhizobium genera plays a role in diversifi cation and in structuring the natural populations of rhizobia.