Recognition of Pyrimidine Dimers in DNA by the Incision Enzyme from <Emphasis Type="Italic">Micrococcus lysodeikticus</Emphasis>

A SIGNIFICANT proportion of the number of pyrimidine dimers induced in DNA by ultraviolet light is repaired by means of excision-resynthesis^1–3. Two enzymes that excise pyrimidine dimers from DNA have been purified from cells of a highly ultraviolet-resistant microorganism—Micrococcus lysodeikticus^4,5. One of these—an endonuclease—seems to recognize dimers and splits a phosphodiester bond near the dimers in DNA. The mechanism of recognition is not known; in particular, whether the incision enzyme recognizes either a local melting of DNA double helix or a specific chemical modification of one of DNA strands. If the former is correct, the incision enzyme should break the strand opposite to the dimer⁶ and the incision step in repair may lead to mutations⁶ or chromosome aberrations⁷.