The most prevalent DNA lesions induced by UVB are the cyclobutane pyrimidine dimers (CPDs) and the pyrimidine (6-4) pyrimidone photoproducts ((6-4)PPs). It has been a long standing controversy as to which of these photoproduct is responsible for mutations in mammalian cells. Here we have introduced photoproduct-specific DNA photolyases into a mouse cell line carrying the transgenic mutation reporter genes lacI and cII. Exposure of the photolyase-expressing cell lines to photoreactivating light resulted in almost complete repair of either CPDs or (6-4)PPs within less than 3 h. The mutations produced by the remaining, nonrepaired photoproducts were scored. The mutant frequency in the cII gene after photoreactivation by CPD photolyase was reduced from 127 ؋ 10 ؊5 to 34 ؋ 10 ؊5 (background, 8 -10 ؋ 10 ؊5 ). Photoreactivation with (6-4) photolyase did not lower the mutant frequency appreciably. In the lacI gene the mutant frequency after photoreactivation repair of CPDs was reduced from 148 ؋ 10 ؊5 to 28 ؋ 10
؊5(background, 6 -10 ؋ 10 ؊5 ). Mutation spectra obtained with and without photoreactivation by CPD photolyase indicated that the remaining mutations were derived from background mutations, unrepaired CPDs, and other DNA photopoducts including perhaps a small contribution from (6-4)PPs. We conclude that CPDs are responsible for at least 80% of the UVB-induced mutations in this mammalian cell model.The UV component of sunlight is responsible for the induction of skin tumors, most notably basal cell and squamous cell carcinomas and probably also melanomas (1, 2). Mutations in the p53 gene have been found in a large percentage of human skin malignancies (3-6). The most frequent p53 mutations in skin tumors are C to T or CC to TT mutations involving dipyrimidine sequences. These are considered characteristic mutational changes that can be ascribed to solar UV irradiation (7).The most abundant UV-induced DNA photoproducts are the cis-syn cyclobutane pyrimidine dimers (CPDs) 1 and the pyrimidine (6-4) pyrimidone photoproducts ((6-4)PPs). Both lesions are produced by UVB (280 -320 nm) and UVC (200 -280 nm) irradiation in DNA (8, 9). Most of the mutagenic and carcinogenic action of sunlight has been ascribed to the UVB portion of the solar spectrum (10) with a possible role for UVA (320 -400 nm) in the induction of melanoma (11). The absorption of UVA photons by DNA is rather weak, and it is thought that indirect damaging mechanisms may involve endogenous chromophores as radiation absorbing intermediates (12). These can generate reactive oxygen species, which may then damage DNA. Of all lesions formed in DNA after UVB irradiation, the CPD is considered one of the most important ones based on its relatively high abundance, slow repair, and known mutagenicity (8, 9, 13). However, a strong case can be made that the (6-4)PPs are equally if not more important than the CPDs for inducing mutations. C to T transitions can be induced by both CPDs and (6-4)PPs in mutagenesis studies using site-specific photolesions, and (6-4)PP...