Respective roles of cyclobutane pyrimidine dimers, (6-4)photoproducts, andminor photoproducts in ultraviolet mutagenesis of repair-deficient xeroderma pigmentosum A cells

The role of UV light-induced photoproducts in initiating base substitution mutation in human cells was examined by determining the frequency and spect rum of mutation in a supF tRNA gene in a shuttle vector plasmid transfected into DNA repair deficient cells (xeroderma pigmentosum complementation gro up A). To compare the role of two major UV-induced photoproducts, cis-syn c yclobutane-type pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone ph otoproducts (6-4PPs), each photoproduct was removed from UV-irradiated plas mid by photoreactivation before transfection, Removal of either CPDs or 6-4 PPs by in vitro photoreactivation reduced the mutation frequency while keep ing the mutation distribution and the predominance of G:C-A:T transitions a s UV-irradiated plasmid without photoreactivation, indicating that both cyt osine-containing CPDs and 6-4PPs were premutagenic lesions for G:CA:T trans itions. On the other hand, A:T-G:C transitions were not recovered from plas mids after the removal of 6-4PPs, whereas this type of mutation occurred at a significant level (11%) after the removal of CPDs, Thus, the premutageni c lesions for the A:T-G:C transition are 6-4PPs. Removal of both CPDs and 6 -4PPs resulted in the disappearance of mutational hot spots and random dist ribution of mutation as observed in unirradiated control plasmids, However, the mutational spectrum of photoreactivated plasmids differed significantl y from that of unirradiated plasmids, A characteristic feature is a high po rtion of A:T-T:A transversions (11%) in the photoreactivated plasmid, This mutation is due to nondipyrimidinic "minor" photoproducts, and the mutation spectrum suggests that TA*, the major photoproduct of thymidylyl-(3'-5')-d eoxyadenosine, is the premutagenic lesion for this mutation. This is the fi rst report revealing the distinct mutagenic roles of the major UV photoprod ucts and "minor" photoproducts by the use of (6-4)photolyase.