Aspects of specific protein-DNA interaction; multi-mode binding of the oligopeptide antibiotic netropsin to (A.T)-rich DNA segments.

By means of titration viscometry a number of distinct modes could be resolved for the interaction between the antibiotic netropsin and DNA species of 50, 58, and 69 mole + (A+T) below r = 0.04 netropsin molecules bound per DNA phosphate group. The number of corresponding binding sites increases with a high power of the (A+T) content. The apparent association constants are very high (greater than 10(6) M-1, some perhaps greater than 10(6) M-1) and also rather different for most of the binding sites. It is suggested that some of these interaction modes differ in the number of hydrogen bonds formed between donors of the ligand and acceptors of the binding sites. The interaction modes were characterized quantitatively by their (species-independent) changes of DNA contour length and by the percentage of local DNA stiffening.