Model nucleoproteins: Binding of actinomycin D to complexes of DNA with lysine‐containing polypeptides

Complexes of DNA with histone H1 and random and sequential polypeptides containing 30–100% of lysine were studied using actinomycin D as a probe. The binding of actinomycin D was measured by spectrophotometric titration in 0.15 M NaCl and in 0.01 M Tris buffer. The excluded‐site model was used for the evaluation of binding data. Polypeptides reduce the number of binding sites on DNA available for actinomycin D binding. The extent of this change depends mainly on the content and distribution of basic lysine residues. Of the hydrophobic residues constituting the peptides, only leucine strongly depresses the actinomycin D binding. The helix‐forming and helix‐breaking amino acid residues are without effect.