RESUMEN
The relationships among selected streptomycetes, nocardiae, and mycobacteria have been determined, based upon the base composition of their deoxyribonucleic acid (DNA) and upon the ability of their denatured DNA to anneal with single-stranded reference DNA. The streptomycetes constituted a homogeneous group whose DNA contained between 69 and 73 mole% guanine + cytosine (% GC). Moreover, the streptomycetes examined showed 37 to 88% homology with the Streptomyces venezuelae and S. rimosus reference DNA. The nocardial and mycobacterial DNA both contained 62 to 69% GC. The nocardial strains studied fell into either a 62 to 64% GC group or a 68 to 69% GC group, indicating that they should not be assigned to a single species. The nocardiae having 68 to 69% GC showed 24 to 44% homology with S. venezuelae reference DNA. In competition experiments, wherein unlabeled heterologous DNA interfered with binding of labeled homologous DNA, the nocardial DNA with 68 to 69% GC showed a greater degree of homology with the streptomycetes than did the nocardial DNA with 62 to 64% GC. In addition, the DNA from spores of S. venezuelae was cursorily examined, and interactions between S. venezuelae denatured DNA and polyribonucleotides were sought. The buoyant density of the DNA from S. venezuelae spores was distinctly less than that from mycelia. Moreover, denatured S. venezuelae DNA formed a dense complex with polyriboguanylate.