SUMMARY: The base composition of DNA preparations from 12 strains of Streptomyces and 1 strain of Nocardia were determined from their denaturation temperature ( ) and buoyant density.

The % GC (guanine+cytosine) contents ranged from 74·4 to 78·5 (from m) and from 69·4 to 73·4 (from buoyant density). The correlation between the two sets of data and the found differences are discussed. The range of pH values which did not affect and the degree of hyperchromic effect have been related to the ionic strength of the solvent. With 0·01 M-phosphate as solvent the pH indifference range was narrower and the hyperchromic effect smaller than with 0·2M-Na solvent.

Though the difference between the maximum and minimum % GC in Streptomyces is sufficient to distinguish the respective strains, within the extreme values there is a continuous progression of base composition which does not permit taxonomic divisions to be made on the basis of overall % GC alone.


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