Intra- and Intergeneric Similarities between the Ribosomal Ribonucleic Acid Cistrons of Rhizobium and Bradyrhizobium Species and Some Related Bacteria

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Abstract
14C-labeled ribosomal ribonucleic acids (rRNAs) were prepared from Bradyrhizobium japonicum NZP 5549T(T = type strain) and Rhodopseudomonas palustris DSM 130, and [3H]rRNAs were prepared from Rhizobium meliloti NZP 4009 and Rhizobium loti NZP 2037. Labeled rRNA from Agrobacterium tumefaciens ICPB TT111 (a member of Agrobacterium cluster 2) was also used. These rRNAs were hybridized under optimal conditions with filter-fixed deoxyribonucleic acids (DNAs) from 24 strains of rhizobia representing nine previously identified DNA-DNA homology groups and with DNAs from various other organisms. Each hybrid was described by the following two parameters: the temperature at which 50% of the hybrid was denatured (Tm(e) ) and percentage of rRNA binding. From rRNA similarity maps and a Tm(e) dendrogram the following conclusions were drawn. (i) There are at least three genetically distinct groups in the present genus Rhizobium. Group I includes Rhizobium meliloti, Rhizobium fredii, and Rhizobium leguminosarum. Group II is represented by Rhizobium loti, and group III includes rhizobia isolated from Galega officinalis and Galega orientalis (goatsrue.). (ii) The rRNA cistrons of group I strains resemble those of Agrobacterium cluster 2 strains. (iii) The rRNA cistrons of Rhizobium fredii resemble those of Rhizobium meliloti more closely than the rRNA cistrons of Rhizobium leguminosarum or Agrobacterium cluster 2 do. (iv) The rRNA cistrons of Rhizobium loti resemble the rRNA cistrons of Rhizobium meliloti less than the rRNA cistrons of strains belonging to either Agrobacterium cluster 1 or Agrobacterium cluster 2. (v) The rRNA cistrons of Galega rhizobia resemble the rRNA cistrons of Rhizobium meliloti about as much as the rRNA cistrons of Agrobacterium cluster 1 and resemble the rRNA cistrons of Rhizobium loti less. (vi) The rRNA cistrons of Rhizobium and Bradyrhizobium resemble one another much less than do the rRNA cistrons of Rhizobium and other genera presently included in the family Rhizobiaceae (Agrobacterium and Phyllobacterium). (vii) Bradyrhizobium strains belonging to four DNA-DNA homology groups all contain rRNA cistrons which closely resembled the rRNA cistrons of Bradyrhizobium japonicum NZP 5549T. (viii) B. japonicum contains rRNA cistrons which have diverged from the rRNA cistrons of Rhodopseudomonas palustris to about the same extent as the rRNA cistrons of Rhizobium group I and Agrobacterium; on this basis we suggest that the photosynthetic ancestor of Bradyrhizobium may have been a rhodopseudomonad and that a phylogenetic classification should group these genera together. (ix) Stemnodulating strain ORS 571, from Sesbania rostrata belongs to the Rhodopseudomonas palustris rRNA branch; it is quite distinct from B. japonicum and Rhodopseudomonas palustris. Two representative strains which nodulate roots belong in the group containing the fast-growing rhizobia.