The distribution of rhizobia, bacteria which nodulate roots of leguminous plants, was surveyed for 91 species of the family Leguminoceae. One hundred and twenty-one strains of rhizobia were isolated, and 29 strains were obtained from culture collections. The total of 150 strains were classified based on their small subunit (ssu) ribosomal RNA (rRNA) sequences. The analyses of the partial sequences (157 bases from positions 1220 to 1377 in Escherichia coli numbering system) revealed the presence of three major groups which corresponded with the genera Rhizobium, Bradyrhizobium, and Azorhizobium and the presence of 17 varieties (eight varieties for the genus Rhizobium, eight for the genus Bradyrhizobium, and one for the genus Azorhizobium). Several phenotypic characteristics and DNA base compositions were determined for 27 representative strains from 17 varieties. Also, genomic relatedness among the 27 strains was estimated by DNA-DNA hybridization. The strains which were different in their ssu rRNA partial sequences never showed close relationships in DNA-DNA hybridization. Further, sequences covering most of ssu rRNA were compared among 17 representative strains of the 17 varieties including type strains of seven Rhizobium species, Bradyrhizobium japonicum and Azorhizobium caulinodans, and phylogenetic relationships among rhizobia were discussed. From phylogenetic analyses it is inferred that the nodulation genes transferred among rhizobia.
The Microbiology Research Foundation