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We prepared hybrids between 14C-labeled ribosomal ribonucleic acid (rRNA) from either Agrobacterium tumefaciens ICPB TT111 or A. rhizogenes ICPB TR7, and deoxyribonucleic acid (DNA) from a great variety of reference gramnegative and gram-positive bacteria. Each hybrid was described by (i) its T m(e) the temperature at which 50% of the hybrid was denatured, and (ii) percentage of rRNA binding, i.e., micrograms of 14C-labeled rRNA duplexed per 100 μg of filter-fixed DNA. Each taxon occupied a definite area on the rRNA similarity map. The size and shape of this area depended on the phenotypic and genetic heterogeneity of the taxon. There appeared to be a correlation between Tm(e) of the heterologous hybrids and the overall phenotypic similarities of the organisms and taxa involved. Tm(e) values above 65°C were taxonomically most meaningful. DNA: rRNA hybridizations condensed all strains from a genus in one narrow cluster; the method had little resolution to distinguish species within a genus, but it seemed to be a very useful approach to detect remote relationships at the inter- and suprageneric level, for taxonomic and identification purposes. The hybrid parameters of Azotomonas fluorescens, Mycoplana bullata, Mycoplana dimorpha, Phyllobacterium, two misnamed “Chromobacterium lividum” strains from leaf-nodulating plants, two misnamed agrobacteria from the Baltic Sea, and a few misnamed “Achromobacter” strains were all in the vicinity of Agrobacterium and Rhizobium. We suggest that all of these organisms are remote relatives and belong in the family of the Rhizobiaceae. Azotomonas insolita NCIB 9749 is misnamed; it is an Agrobacterium. Several organisms which had been misnamed Agrobacterium formed DNA: rRNA hybrids with properties outside the Agrobacterium area.
Smedt et al. (Fri,) studied this question.