DNA binding properties of minor groove binders and their influence on the topoisomerase II cleavage reaction.

We present titrations of the human delta beta-globin gene region with DNA minor groove binders netropsin, bisnetropsin, distamycin, chromomycin and four bis-quaternary ammonium compounds in the presence of calf thymus topoisomerase II and DNase I. With increasing ligand concentration, stimulation and inhibition of enzyme activity were detected and quantitatively evaluated. Additionally we show a second type of stimulation, the appearance of strong new topoisomerase II cleavage sites at high ligand concentrations. The specific binding sites of the minor groove binders of the DNA sequence and their microscopic binding constants were determined from DNase I footprints. A binding mechanism for minor groove binders is proposed in order to explain these results especially when ligand concentration is increased.