The (6-4) photoproduct of thymine-thymine induces targeted substitution mutations in mammalian cells.

Two major ultraviolet-induced photolesions of TpT, a (6-4) photoproduct [T(6-4)T] and a cis-syn cyclobutane TT dimer (T=T), were incorporated into a predetermined site of one of the leading and lagging template strands of a double-stranded vector, and the modified DNAs were transfected into simian COS-7 cells. The DNAs replicated in the cells were recovered and were transfected again into Escherichia coli. The DNA replication efficiencies of plasmids containing T(6-4)T and T=T in the template strand for lagging strand synthesis were 93 and 79%, respectively, as compared with the unmodified DNA. Similar inhibitory effects were observed in the cases of the photoproducts in the template strand for leading strand synthesis (71 and 58%, respectively). These results indicated that T(6-4)T blocked DNA replication more weakly than T=T during leading and lagging strand syntheses in mammalian cells. The mutation frequencies of T(6-4)T were 2.3 and 4.7% in the leading and lagging template strands, respectively. The T=T lesion was less mutagenic and induced mutations with 0.2-0.7% frequencies. The T(6-4)T lesion primarily elicited 3'-T-->C substitutions, and T=T induced various types of mutations. These results indicate that T(6-4)T is more mutagenic than T=T during leading and lagging strand syntheses in simian cells. Moreover, this is the first evidence that shows T(6-4)T mainly elicits targeted substitutions at its 3'-T site in mammalian cells.