Excision of cyclobutane dimers in genomic and episomal DNA in human cells.

Direct determination has been made of cyclobutyl pyrimidine dimer induction and excision repair in an episomal SV40 DNA population in vivo. Maintaining SV40-transformed human (GM637) cells in confluent culture results in amplification of a mutant SV40 episome to high copy number. T4 endonuclease V was used to quantify the induction and repair of cyclobutane dimers in the SV40 episome and genomic DNA of the same cells. Differences in both parameters were observed; cyclobutane dimers were induced at 1.5-2-fold greater frequency in episomal DNA and excised at a reduced rate compared to genomic DNA in the host cells.