Deoxyribonucleic acid homology in bacterial taxonomy: effect of incubation temperature on reaction specificity.

Parameters affecting deoxyribonucleic acid duplex (DNA-DNA) formation on membrane filters were evaluated. The reference strains used were Cytophaga succinicans strain 8, which has a guanine plus cytosine (GC) content of 38%, and Myxococcus xanthus strain FB, which has a GC content of 70%. Both organisms are gliding bacteria classified among the myxobacteria. Among the parameters evaluated, the incubation temperature used during duplex formation was found to be the most important in terms of the physical nature of the reaction product. When an incubation temperature 25 C below the melting point (T(m)) of the native DNA was used, homologous duplexes exhibited a thermal stability similar to that of native DNA. At 35 C below the T(m), a considerable proportion of the duplexes were of much lower stability; at 40 C below the T(m), most of the duplexes were of much lower stability. Similar duplexes of low stability were also formed between DNA molecules from morphologically and nutritionally diverse organisms, provided the GC percentages of the DNA preparations were similar. Competition between unlabeled and labeled DNA fragments for binding sites on immobilized DNA was also greatly influenced by the incubation temperature. Heterologous DNA-DNA complexes exhibited thermal stabilities which correlated with measurements of DNA homology in experiments involving competition. In addition, the difference in thermal stabilities of heterologous and homologous DNA complexes (DeltaT'(m)) may provide a measure of divergence in nucleotide sequences.