Map of distamycin, netropsin, and actinomycin binding sites on heterogeneous DNA: DNA cleavage-inhibition patterns with methidiumpropyl-EDTA.Fe(II).

We report a direct technique for determining the binding sites of small molecules on naturally occurring heterogeneous DNA. Methidiumpropyl-EDTA.Fe(II) [MPE.Fe(II) cleaves double helical DNA with low sequence specificity. Using a combination of MPE.Fe(II) cleavage of drug-protected DNA fragments and Maxam-Gilbert gel methods of sequence analysis, we have determined the preferred binding sites on a Rsa I-EcoRI restriction fragment from pBR322 for the intercalator actinomycin D and the minor groove binders netropsin and distamycin A. Netropsin and distamycin A gave identical DNA cleavage-inhibition patterns and bound preferentially to A+T-rich regions with a minimal protected site of four base pairs. We were able to observe the effect of increasing concentration on site selection by netropsin and distamycin A. Actinomycin D afforded a completely different cleavage-inhibition pattern, with 4- to 16-base-pair-long protected regions centered around one or more G.C base pairs.