Helix-coil transition in nucleoprotein: renaturation of polylysine-DNA and polylysine-nucleohistone complexes.

Abstract

AbstractThermal denaturation and renaturation of directly mixed and reconstituted polylysine–DNA, directly mixed polylysine–nucleohistone complexes, and NaCl‐treated nucleohistones in 2.5 × 10−4 M EDTA, pH 8.0 have been studied. At the same input ratio of polylysine to DNA, the percent of renaturation of free base pairs in a directly mixed polylysine–DNA complex is higher than that in a reconstituted complex. For a directly mixed complex, the renaturation of free base pairs is proportional to the fraction of DNA bound by polylysine or inversely proportional to the sizes of free DNA loops. A of large amount of renaturation of free base pairs has also been observed for 0.6 M and 1.6 M NaCl‐treated nucleohistones. The binding of polylysine to nucleohistone enhances the renaturation of histone‐bound base pairs. The percent of renaturation of polylysine–bound base pairs is high and is approximately independent of the extent of binding on DNA by polylysine. This is true in polylysine–DNA complexes prepared either by reconstitution or by directly mixing. It also applies for polylysine–nucleohistone complexes.The model where polylysine‐bound base pairs collapse at Tm′ with two complementary strands still bound by polylysine is favored over the model where polylysine is dissociated from DNA during melting. The low renaturation of histone‐bound base pairs in nucleo‐histone indicates that either histones do not hold two complementary strands of DNA tightly or that histones are fully or partially dissociated from DNA when the nucleo‐histone is fully denatured.