Thermal denaturation of calf thymus DNA: Existence of a GC-richer fraction

Abstract

In 2.5 × 10-4M EDTA buffer, the derivative melting curve of calf thymus DNA shows a major band at 47° with a shoulder at about 54°. The fraction of melting area of this shoulder is about 13%. For reconstituted polylysine-calf thymus DNA complexes, in addition to the melting of free DNA regions at about 50° (Tm) there is another melting at about 106° (T′m) of polylysine-bound repions. The melting band of the complex at Tm is not symmetrical. As more polylysing is bound to DNA the melting amplitude is diminished greatly on the major band at 4° but only slightly on the shoulder at 54°. The insensitivity of this shoulder appears to result from the existence of a 13% fraction of calf thymus DNA containing 55% GC. It is not favorably bound by polylysine. It remains in the supernatant after centrifugation and melts at about 54-56°. This conclusion is further supported by two facts: the reconstitution method provides a condition for selective binding of polylysine to AT-rich DNA, and it yields a fully symmetric melting band at Tm for complexes of pclylysine with homogeneous bacterial DNA such as the one from M. lutous. © 1974 Oxford University Press.