The influence of chromatin structure on the distribution of DNA repair synthesis studied by nuclease digestion.

The influence of chromatin structure on the distribution of DNA repair synthesis was studied by enzymatic digestion of "repair labeled" nuclei of mouse mammary cells: "repair labeled" nuclei were isolated from pregnancy mammary tissue fragments, treated in vitro with methylmethanesulfonate (MMS) or methylnitrosourea (MNU), and pulse-labeled with 3H-thymidine in the presence of hydroxyurea in the culture medium. Micrococcal nuclease digestion of "repair labeled" nuclei indicates that at early hours after treatment with the alkylating agents 70-80% of the total repair synthesis is located in the linker portion of the nucleosome. However, 6-12 hours after treatment DNA repair synthesis is more evenly distributed throughout the core and linker portion of the nucleosome. "Repair labeled" mammary cell nuclei were also digested with DNase I under conditions selective for transcriptionally active chromatin. A two-fold higher level of repair synthesis was found in the transcriptionally active chromatin of "repair labeled" nuclei isolated from MMS or MNU treated mammary fragments, pulse-labeled at different times after treatment. The results indicate that structural constitution of the chromatin may influence the distribution of DNA repair synthesis both at the nucleosome level, and at higher levels of chromatin organization. This may be due to 1) nonrandom base alkylation in chromatin or 2) areas in chromatin with increased accessibility for the repair enzymes to the alkylated bases.