Nucleosome rearrangement in human chromatin during UV-induced DNA- reapir synthesis

The distribution of UV-induced DNA repair synthesis within chromatin was measured in confluent human fibroblasts that were pulse-labeled with [3H]dThd (10 or 90 min) immediately after irradiation and chased in nonradioactive medium for different time periods. Initially (i.e., at the end of the pulse period), most of the repair synthesis occurs in staphylococcal nuclease-sensitive regions. With increasing chase times the nucleotides inserted during repair synthesis become progressively more nuclease resistant. Gel electrophoresis data indicate that nuclease resistance is conferred on these nucleotides by their appearance in core DNA. The kinetics of this rearrangement process are biphasic: greater than 85% of the repair synthesis sites undergo rapid rearrangement (4--5 hr); the remaining sites ( less than 15%) rearrange much more slowly, if at all. The time courses of nucleosome rearrangement and repair synthesis are similar, suggesting that nucleosome rearrangement may be induced by the repair process or that the rate of repair synthesis may be regulated by nucleosome rearrangement.