The Interaction of Chromatin with Alkylating Agents

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Abstract
The reaction of L5178Y lymphoblast cell chromatin with the alkylating agent bis(2‐chloroethyl)‐methylamine has been studied as a function of time, pH and reagent concentration. The reaction with DNA of chromatin from which the proteins were dissociated, as well as with purified calf thymus DNA, was studied in parallel. The extent of alkylation of DNA in intact chromatin was 4–5 times as much as in parallel free DNA samples; up to 4% of nucleotide base pairs were substituted. The extent of monofunctional substitution of the proteins was similar, on a weight basis, to that of DNA. Chromatographic analysis of the depurinated products showed that in chromatin, as in DNA, position N‐7 of guanine is the major site of reaction. Up to 25% of the reaction products were guanines cross‐linked as bis(2‐guanin‐7‐yl‐ethyl)methylamine, indicating a considerable degree of DNA‐DNA cross linking. Column analysis shows that up to 40% of the nuclear proteins are cross‐linked to DNA at 10 mM bis(2‐chloroethyl)methylamine. The increased reactivity of intact chromatin is interpreted in terms of a conformational change in the position of the DNA bases when in the organized nucleohistone complex.