Early steps of excision repair of cyclobutane pyrimidine dimers by the Micrococcus luteus endonuclease. A three-step incision model.

The early steps of excision repair of cyclobutane pyrimidine dimers are investigated. It is demonstrated that the apurinic/apyrimidinic endonuclease associated with the Micrococcus luteus uv-specific endonuclease cleaves the phosphodiester bond on the 3' side of the deoxyribose leaving a 3' hydroxy terminus and a 5' phosphoryl terminus. This nick is not a substrate for T4 polynucleotide ligase. The 3' base-free deoxyribose terminus is not a substrate for either the polymerase or the 3' to 5' exonuclease activities of Escherichia coli DNA polymerase I. However, the 3' terminus of the nick is converted to a substrate for DNA polymerization by the action of a 5' apurinic/apyrimidinic endonuclease. A three-step model for the incision step of excision repair of cyclobutane pyrimidine dimers is presented.