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-pyrimidine sequences, and terminal 5'- and 3'-thymines; this process can be reversed by photoreactivation. The UV-induced 160-mers are sensitive to digestion by the restriction enzyme SnaBI, but monomers are insensitive to digestion, indicating that UV irradiation stabilizes the formation of double-stranded DNA. These results suggest that UV irradiation of these 80-mer oligonucleotide substrates induces the formation of a novel cyclobutane thymine dimer which lacks an intradimer phosphodiester bond (CPD*). This CPD*, linking the terminal thymines of two separate 80-mer molecules, is formed in a double-stranded DNA region created 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 dimer-specific enzyme T4 endonuclease V. However, pretreatment of the 160-mers with ligase reverses their sensitivity to these two enzymes, significantly reducing their susceptibility to cleavage by UvrABC nuclease but dramatically increasing their susceptibility to cleavage by T4 endonuclease. The biological significance of these findings is discussed.