Electron-Microscopic Visualization of the AAF Binding Sites to DNA in Core Particles and Trinucleosomes by Means of Specific Antibodies
The covalent binding of the so-called ultimate metabolite of a given carcinogen to DNA is generally believed to be the initial step in chemical carcinogenesis. In the field of aromatic amides or amines carcinogenesis, N-2-acetylaminofluorene (AAF) is the most widely studied compound. In vitro studies are conducted with the model ultimate metabolite N-acetoxy-N-2-acetylaminofluorene (N-Aco-AAF). This compound is known to bind covalently to DNA giving rise to nucleoside — AAF adducts of the same nature as those observed under in vivo conditions1. N-2-(deoxyguanosine-8-y1)-acetylaminofluorene (dGuo-C8-AAF), the major adduct, has been shown to induce a structural alteration of the DNA double helix which has been described in this laboratory as the insertion-denaturation model2,3. In eukaryotic cells DNA rarely appears free but associated with proteins inside the structure of the chromatin. We asked therefore the question of how this structure would interfere with the covalent binding of N-Aco-AAF to DNA. To study this problem we used a methodology which has been recently developped in our laboratory4 and which consists in the direct electron-microscopic visualization of the AAF binding sites along the DNA molecule by means of specific antibodies. In fact, native DNA modified with N-Aco-AAF5 as well as Guo-C8-AAF linked to BSA6,7 induce in rabbits the synthesis of antibodies that specifically recognize the AAF modified guanine residue in native DNA7,8. These specific antibodies have been purified from the crude serum 7,8 and subsequently used to map the AAF binding sites along the DNA fragments isolated from core particles and trinucleosomes.
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