Abstract
The extent of association between deoxyribonucleic acid (DNA) from various actinomycetes with reference DNA samples from two strains of Nocardia erythropolis has been used as an index of relatedness. N. erythropolis, N. canicruria, and particular cultures designated as N. corallina and N. opaca constitute a genospecies, on the bases of ability to form DNA hybrid molecules and ability to participate in genetic recombination. The lack of reciprocity in the DNA reassociation assays involving selected strains of N. erythropolis indicates that the DNA from particular lineages possesses unique nucleotide sequences. The taxon Mycobacterium rhodochrous is heterogeneous and contains at least two discrete populations. The present results do not clarify the relationships of the N. erythropolis genospecies to the other nocardiae, streptomycetes, or mycobacteria. The differences in extent of hybrid DNA duplex formation at exacting and non-exacting incubation temperatures reflect the degree of matching between the compared nucleotide sequences. On this basis, actinomycete DNA samples differ from one another by nucleotide alterations dispersed throughout the genome. DNA hybridization is a sensitive assay for molecular evolution and may be able to detect neutral and nearly neutral mutations that are not recognized phenotypically.
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