Nucleotide excision repair of DNA by human cell extracts is suppressed in reconstituted nucleosomes

Nucleotide excision repair is a complex biochemical process that corrects DNA damage caused by a broad spectrum of physical and chemical agents. We examined the effect of the assembly of ultraviolet-irradiated plasmid DNA into nucleosomes on nucleotide excision repair supported by human cell extracts. Repair synthesis in unassembled UV-irradiated plasmid DNA was readily detected in extracts from repair-proficient human cells. In contrast, repair synthesis was markedly suppressed in UV-irradiated DNA assembled into nucleosomes (minichromosomes). This suppression occurred at a step(s) which precedes repair synthesis during nucleotide excision repair. Human cell extracts were unable to effectively assemble plasmid DNA into nucleosomes under repair synthesis conditions. The addition of purified histones to the extracts restored their capacity for nucleosome assembly and simultaneously led to the suppression of repair synthesis. We propose that the preferential repair of actively transcribed genes relative to transcriptionally silent genes may reflect altered nucleosome conformation during transcription.