Ultraviolet-induced dimerization of non-adjacent pyrimidines. A potential mechanism for the targeted -1 frameshift mutation.

The DNA photoproduct responsible for the ultraviolet (u.v.)-induced targeted -1 frameshift mutation is unknown. Based on mutagenesis studies by others, we surmised that this lesion might be found in high abundance in single-stranded DNA. u.v. irradiation of the single-stranded alternating copolymer poly[d(G-T)] yielded a photoproduct that was characterized in detail. It consists of a thymine-thymine cyclobutane dimer of predominantly cis-syn configuration occurring between non-adjacent thymidyl residues on the same strand. Its formation is strongly inhibited in double-stranded DNA. A similar u.v. photoproduct was obtained in higher yield from the polypyrimidine alternating copolymer poly[d(C-T)] under conditions in which it is single-stranded. It is proposed that replication across the unrepaired photoproduct: (formula; see text) is the cause of the targeted u.v.-induced -1 frameshift mutation.