Chicken erythrocyte chromatin and nuclei were labeled with benzo[a]pyrene (B[a]P) diol-epoxide (anti) and digested with micrococcal nuclease to mono- and dinucleosomes. Analysis of the distribution of the carcinogen showed that the internucleosomal region bound 3-4 times more carcinogen per unit DNA than did nucleosomes. The enhanced binding of the 'ultimate' carcinogen to the internucleosomal region was similar when isolated chromatin or nuclei were used for in vitro labeling. Furthermore, isolation of the histone core proteins, H2A, H2B, H3 and H4, revealed that only 15% of the carcinogen was associated with the histones and that the majority of the carcinogen was bound to chromosomal DNA. Fluorography of purified nucleosomal histones showed that the covalent association of the carcinogen was mainly with histones H3 and H2B.
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