EFFECT OF NUCLEOSOME STRUCTURE ON DNA INTERSTRAND CROSS-LINKING REACTIONS

Antitumor agents of the nitrogen mustard family and mitomycin C form i nterstrand crosslinks in duplex DNA. To provide information about the cellular mechanism by which these compounds exert their cytotoxic effe cts, we examined cross-linking of a nucleosomal core particle formed o n a fragment of the 5S RNA gene of Xenopus borealis. For the mustards mechlorethamine, chlorambucil, and melphalan, both sites of monoalkyla tion and interstrand cross-linking were similar in nucleosomal and fre e DNA. Some small (two-to three-fold) differences in intensity of cros s-linking at some sites were apparent. However, these differences did not appear to correlate with rotational or translational positioning. For mitomycin C, cross-linking was inhibited five-to ten-fold at the n ucleosomal dyad and showed attenuation of inhibition toward the ends. Furthermore, rotational positioning also appeared to be a factor, with sites facing inward in the nucleosome less accessible for mitomycin c ross-linking. None of these agents demonstrated the 10-base pair perio dicity exhibited by hydroxyl radical cleavage of nucleosomal DNA.