Cellular responses and repair of single-strand breaks introduced by UV damage endonuclease in mammalian cells

Although single-strand breaks (SSBs) occur frequently, the cellular respons es and repair of SSB are not well understood. To address this, we establish ed mammalian cell Lines expressing Neurospora crassa UV damage endonuclease (UVDE), which introduces a SSB with a 3'-OH immediately 5' to UV-induced c yclobutane pyrimidine dimers or 6-4 photoproducts and initiates an alternat ive excision repair process. Xeroderma pigmentosum group A cells expressing UVDE show UV resistance of almost the wild-type level. In these cells SSBs are produced upon UV irradiation and then efficiently repaired. The repair patch size is about seven nucleotides, and repair synthesis is decreased t o 30% by aphidicolin, suggesting the involvement of a DNA polymerase delta/ is an element of -dependent long-patch repair. Immediately after UV irradia tion, cellular proteins are poly(ADP-ribosyl)ated. The UV resistance of the cells is decreased in the presence of 3-aminobenzamide, an inhibitor of po ly(AI)P-ribose) polymerase. Expression of UVDE in XRCC1-defective EM9, a Ch inese hamster ovary cell line, greatly sensitizes the host cells to UV, and addition of 3-aminobenzamide results in almost no further sensitization of the cells to UV. Thus, we show that XRCC1 and PARP are involved in the sam e pathway for the repair of SSBs.