Excision of Pyrimidine Dimers from Ultraviolet‐Irradiated DNA by Exonucleases from Mammalian Cells

Two DNA specific exonucleases, DNase III and Dnase IV, are present in mammalian cell nuclei. The activity of the purified enzymes with ultaviolet‐irradiated polydeoxynucleotides and DNA as substrates has been investigated. DNase III shows a very poor ability to release pyrimidine dimers, althought excision can be demonstrated by using high concentrations of enzyme. In contrast, DNase IV liberates dimers in an effective fashion in vitro, and is a likely candidate to serve in this function also in vivo. The pyrimidine dimers are relesed by DNase IV as parts of oligonucleotides containing 5–8 residus. Neither of the two exonucleases is markedly inhibited by caffeine.