A non-replicating recombinant adenovirus, Ad5MCMVlacZ, which expresses the 13-galactosidase (l3-gal) reporter gene, was used to examine both constitutive and inducible repair of UVC-damaged DNA in Chinese hamster ovary (CHO) cells. Host cell reactivation (HCR) of 13-gal activity for UVC-irradiated Ad5MCMVlacZ was examined in non-irradiated and UVC-irradiated nucleotide excision repair (NER) proficient parental CHO-AA8 and m mutant CHO-UV61 cells which are deficient in the transcription-coupled repair (TCR) pathway of NER. Cells were infected with either UVC-irradiated or non-irradiated Ad5MCMVlacZ and scored for 13-gal activity 24 h later. HCR of 13-gal activity for UVC-irradiated Ad5MCMVlacZ was significantly reduced in non-irradiated CHO-.UV61 cells compared to that in non-irradiated CHO-AA8 cells suggesting that repair in the transcribed strand of the UVC-damaged reporter gene in untreated CHO-AA8 cells utilizes TCR. Prior UVC-irradiation of cells with low UV fluences resulted in a transient enhancement of HCR for expression of the UVC-damaged reporter gene in CHO-AA8 cells but not m TCR deficient CHO-UV61 cells. Pre-UVC-treatment of cells resulted also in an enhanced expression of 13 -gal for unirradiated Ad5MCMVlacZ in both CHO-AA8 and CHO-UV61 cells. However, compared to CHO-AA8 cells, the CHO-UV61 cells exhibited comparable levels of enhanced 13-gal activity following significantly lower UVC exposures to cells suggesting that persistent damage in active genes plays a direct role in enhancing 13-gal activity driven by the MCMV promoter in CHO cells. These results suggest that prior UVC treatment results in a transient enhancement in repair of UVC-damage DNA in the transcribed strand of the active reporter gene in CHO-AA8 cells through an enhancement of TCR or a mechanism that involves the TCR pathway and that the upregulation of reporter gene expression alone is not sufficient for enhanced repair of the reporter gene in CHO-UV61 cells. The HCR assay was used also to examine both constitutive and inducible repair of UVC-damaged DNA in mouse embryonic fibroblast (MEF) cells. HCR of B-gal activity for UVC-irradiated Ad5MCMVlacZ was examined in non-irradiated and UVC-irradiated NER proficient parental wild type MEF cells and in MEF cells with specific knockouts in the p53 (p53-/-), pRb (pRb-/-), and p107 (p107-/-) genes. Cells were infected with either UVC-irradiated or non-irradiated Ad5MCMVlacZ and scored for ~-gal activity 24 h later. HCR of ~-gal activity for UVC-irradiated Ad5MCMVlacZ did not show a significant difference in non-irradiated cells for any of the MEF knockouts cells compared to the parental strain suggesting that p53, pRb and p107 does not play a role in repair of the UV -damaged reporter gene in untreated MEF cells. Prior UVC-irradiation of cells with low UVC fluences resulted in an enhancement of HCR for expression of the UV C-damaged reporter gene in MEF wild type cells, low passage pRb-/-and p 1 07 -I-MEF cells but not in p53-/-MEF cells or in high passage pRb-/-and p107-/-MEF cells. These results suggest that prior UVC treatment MEF cells results in an induced repair of UVC-damaged DNA that is dependent on p53. The presence of an enhancement of HCR for the UVC-damaged reporter gene in pre-UVC treated cells in low passage, but not in high passage, pRb-/-and p 1 07-I-cells suggests that the lack of pRb or pI 07 expression per-se does not result in a deficiency in inducible DNA repair. However, these results suggest that the lack of pRb or p 1 07 expression results in alterations in MEF cells at high passage number that abrogate inducible repair of UVC-damaged DNA. UVA produces predominantly single base damage that is repaired through base excision repair (BER), whereas UVC and UVB produce predominantly cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4PP) that are repaired through NER. The colony survival following exposure to various UV sources was examined in cells proficient and deficient in (NER). The UV sources were a UVC source from a germicidal lamp emitting predominantly at 254 nm. and a UVA source from a lKW Hg-Xe arc lamp using either a Band pass filter (BPF) or a 335 Cut-off-filter (335COF). NER deficient CHO-UV5 and CHO-UV61 cells were more sensitive to UVC exposure compared to NER proficient CHO-AA8 cells, consistent with the production of UVC-induced DNA damage predominantly in the form of CPDs and 6-4PPs which are repaired through the NER pathway. NER deficient xeroderma pigmentosum cells from complementation group D (XPD) were more sensitive compared to NER proficient normal human cells following exposure to the UVA-BPF source. In addition XPDdenV cells, which express the denY gene from bacteriophage T4, were more resistant than XPD cells following exposure to the UVA-BPF source. Since the denY protein is specific for excision ofCPDs these results indicate a substantial proportion of the induced DNA damage resulting from the UV A-BPF is in the form of CPDs, presumably due to a significant UVB component in the beam. In contrast, the NER deficient CHO-UV5 and CHO-UV61 cells showed a similar sensitivity compared to the NER proficient CHO-AA8 cell line following UVA-335COF exposures up to 60 KJ/m2• However, for UVA-335COF exposures greater than 60 KJ/m2 the NER deficient cells were more sensitive compared to the NER proficient CHO-AA8 cells, although the difference in sensitivity between NER deficient and NER proficient cells was less than that detected following UV A-BPF exposure. These results suggest that the UVA-335COF exposure produces predominantly DNA damage of the single base type for exposures less 60 KJ/m2. This is consistent with the calculated spectral distribution, which showed a 5.62% UVB component for the UVA-BPF, but only 0.14% UVB component for the UVA-335COF. / Thesis / Master of Science (MS)