Selective binding of actinomycin D induces a reversible conformational transition of nucleosomes

Equilibrium and hydrodynamic studies on the complex of actinomycin D with H1-H5 depleted, 175 basepair nucleosomes are reported. By spectral titration the intrinsic affinities of actinomycin D for nucleosomes and for DNA are found strictly comparable. Sedimentation analysis shows that actinomycin can apparently unfold the nucleosome, like ethidium bromide and daunomycin, but it does so at a much lower bound drug to DNA molar ratio (about 1 drug molecule to 45 basepairs). Since about four bound actinomycin molecules are able to induce the reversible conformational transition of a nucleosome, it is suggested that the sites of interaction may correspond to the kinked DNA sites evidenced by Klug and collaborators (Richmond, T.J., Finch, J.T., Rushton, B., Rhodes, D. and Klug, A. (1984) Nature 311, 532-537) in the structure of the nucleosome. A relevance of these findings to the interaction of actinomycin with "active chromatin" is also suggested.