ACCESSIBILITY OF DEOXYRIBONUCLEIC-ACID IN CHROMATIN TO THE COVALENT BINDING OF THE CHEMICAL CARCINOGEN BENZO[A]PYRENE

A model system utilizing rat liver microsomes to activate [3H]benzo[a]pyrene (BP) in the presence of calf thymus nuclei was used to examine the ability of BP to bind regions of DNA which differ in their accessibility in chromatin. [3H] BP-modified nuclei were digested with staphylococcal nuclease and DNase I, and the specific activity (cpm of [3H]BP/A260 of DNA) of the DNA remaining undigested was determined. Both enzymes resulted in characteristic changes in specific activity as a function of digestion. No changes occurred during digestion of isolated [3H]BP-DNA, and BP had no effect on the kinetics of digestion of DNA or nuclei, indicating that the specific activity changes seen in nuclear digests were due to preferential binding to DNA in regions of chromatin differing in enzyme susceptibility. The nucleosomal sites of [3H]BP binding were determined by electrophoretic analysis of the resistant DNA and by examining the specific activity as a function of digestion of (1) nucleosome multimers isolated by sucrose gradient sedimentation of [3H] BP-modified nuclei partially digested with staphylococcal nuclease and of (2) monomer subfractions obtained by KC1 precipitation of H1-containing monomers. In addition, the distribution of [3H]BP in fragments obtained from a DNase I digest of nuclei was compared to that of an isolated monomer fraction. These data led to the conclusion that BP binds to the spacer region and the outermost portions of the nucleosome core. © 1979, American Chemical Society. All rights reserved.