We prepared hybrids between C-labeled ribosomal ribonucleic acid (rRNA) from either ICPB TT111 or ICPB TR7, and deoxyribonucleic acid (DNA) from a great variety of reference gramnegative and gram-positive bacteria. Each hybrid was described by (i) its the temperature at which 50% of the hybrid was denatured, and (ii) percentage of rRNA binding, i.e., micrograms of C-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 of the heterologous hybrids and the overall phenotypic similarities of the organisms and taxa involved. values above 65C 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 , two misnamed “” strains from leaf-nodulating plants, two misnamed agrobacteria from the Baltic Sea, and a few misnamed “” strains were all in the vicinity of and . We suggest that all of these organisms are remote relatives and belong in the family of the NCIB 9749 is misnamed; it is an . Several organisms which had been misnamed formed DNA: rRNA hybrids with properties outside the area.


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