CYCLOBUTANE THYMINE DIMERS WITH A DISRUPTED PHOSPHODIESTER BOND ARE REFRACTORY TO T4 ENDONUCLEASE-V DIGESTION BUT HAVE INCREASED SENSITIVITY TO UVRABC NUCLEASE

UV irradiation induces the dimerization of synthetic single-stranded, 80-mer oligonucleotides with self-complementary, alternating purine-py rimidine sequences, and terminal 5'- and 3'-thymines; this process can be reversed by photoreactivation. The UV-induced 160-mers are sensiti ve to digestion by the restriction enzyme SnaBI, but monomers are inse nsitive to digestion, indicating that UV irradiation stabilizes the fo rmation of double-stranded DNA. These results suggest that UV irradiat ion of these 80-mer oligonucleotide substrates induces the formation o f a novel cyclobutane thymine dimer which lacks an intradimer phosphod iester bond (CPD). This CPD*, Linking the terminal thymines of two se parate 80-mer molecules, is formed in a double-stranded DNA region cre ated by self-annealing and intermolecular hybridization of the two 80- mer strands. We have found that these UV-induced CPD in 160-mers are sensitive to cleavage by the nucleotide excision enzyme complex UvrABC nuclease, but resistant to cleavage by the cyclobutane pyrimidine dim er-specific enzyme T4 endonuclease V. However, pretreatment of the 160 -mers with ligase reverses their sensitivity to these two enzymes, sig nificantly reducing their susceptibility to cleavage by UvrABC nucleas e but dramatically increasing their susceptibility to cleavage by T4 e ndonuclease. The biological significance of these findings is discusse d.