Ultraviolet stimulation of intermolecular homologous recombination in Chinese hamster ovary cells

We previously showed that ultraviolet (UV) irradiation of cotransfected plasmid DNA molecules stimulated genetic transformation that depended on intermolecular homologous recombination in Chinese hamster ovary (CHO) cells. Repair‐proficient cells and an excision repair complementation class 1 (ERCC1) UV‐sensitive DNA repair‐deficient mutant responded similarly to UV stimulation in cotransfections with plasmids containing linker insertion‐disrupted copies of the herpes simplex virus thymidine kinase (HSV‐TK) gene. In this study, we cotransfected homologous DNA molecules containing nonoverlapping deletions of the hamster adenine phosphoribosyltransferase (APRT) gene into APRT‐deficient CHO ERCC1 (UVL‐10) and ERCC2 (UVL‐1) excision‐repair mutants and parental repair‐proficient CHO cells. UV damage in cotransfected circular plasmid molecules stimulated transformation in repair‐proficient cells and an ERCC1 mutant, but not in an ERCC2 mutant. Linearization of plasmids prior to cotransfection greatly enhanced transformation frequencies in all three cell lines, but UV stimulation using linear recombination substrates was no longer evident. Our results suggest (i) that the ERCC1 gene defect in CHO UVL‐10 cells does not affect UV stimulation of homology‐dependent extra‐chromosomal recombination, and (ii) that a CHO cell ERCC2 excision‐repair mutant, although recombination proficient, may exhibit altered recombination in response to UV damage.