Isolement et propriétés des chaînes du DNA de bactériophage 2 C

Bacillus subtilis phage 2 C DNA is double‐stranded, with a high molecular weight (about 100 million), a low T m (77.8%), and a high buoyant density (1.742 g/cm 3 ) in a CsCl density gradient. Denatured DNA centrifuged in a CsCl gradient reveals two bands (1.752 and 1.762 g/cm 3 ) corresponding to the complementary strands. In this communication, we report preparative separation of two strands from denatured DNA, either by chromatography on a methylated albumin‐kieselguhr column, or by density‐gradient centrifugation in the presence of poly G. The melting curves of native and denatured DNA, and of isolated fractions before and after mixing and annealing, are compared. As shown by spectrophotometry, and by buoyant density, a good renaturation was obtained by mixing and annealing the “light” and “heavy” fractions; each isolated fraction cannot be renatured alone. Poly G strongly binds to the heavy fraction, increasing its density in a CsCl gradient by about 48 mg/cm 3 ; there is also some binding to the light fraction (density increase under the same conditions, 6 mg/cm 3 ). The resulting density difference between the two fractions is more pronounced than the natural one: 52 mg/cm 3 instead of 10 mg/cm 3 . Poly C only interacts with the light fraction and host ribosomal RNA with the heavy one. Native DNA, and both fractions isolated by centrifugation in the presence of poly G, were analysed for base composition using 32 P‐labelled DNA. Native DNA had a (G + C) content of 39.1%. As for the isolated fractions, there are more pyrimidines (cytosine, and hydroxymethyluracil replacing thymine) in the “heavier” fraction, which preferentially interacts with poly G. Both strands from infectious DNA were assayed for infectivity with negative results. The “renatured” DNA also gave negative results, probably owing to some natural or accidental breakage.