Binding of chlorozotocin and 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea to chromatin and nucleosomal fractions of HeLa cells.

Both chlorozotocin and 1-(2-chloroethyl)-3-cyclohexyl-1nitrosourea (CCNU) bind specifically to the extended euchromatin fraction of the HeLa cell genome. After a 2-hr incubation of log-phase cells with each radiolabeled drug at 30 /(M, both alkylation (chlorozotocin and CCNU) and carbamoylation (CCNU) of nuclear chromatin were shown to be confined to late-eluting fractions from an ECTHAMcellulose chromatography column. These data were con firmed by enzymatic digestion of drug-treated nuclei with pancreatic DNase I (preferentially cleaves transcriptionally active chromatin). By following the kinetics of diges tion, the alkylated chromatin was preferentially solubilized by the enzyme. In addition, when compared to micrococcal nuclease digestions, both chlorozotocin and CCNU were shown to alkylate the DNA associated with the nucleosome core particle. Quantitatively, chromatin alkylation by chlorozotocin was twice that of CCNU. Pretreatment of log-phase cells with 5.0 HIM sodium butyrate affected the chromatin struc ture by causing historie modifications and an increase in transcriptional activity. Concomitantly, the cellular uptake of both nitrosoureas was increased twofold. Also, the chromatin alkylation and historie and nonhistone protein carbamoylation were increased twoto threefold. At a concentration of 30 UM CCNU, carbamoylation of nuclear proteins was restricted to the nonhistone frac tions. Alkylation of histone proteins was detected only when a chromatin suspension (prepared by mild digestion of nuclei with micrococcal nuclease) was treated with 0.3 mil CCNU.