Excision of thymine dimers in vitro by extracts of bacteriophage-infected Escherichia coli.

Extracts of DNA polymerase I defective Escherichia coli infected with phage T4 contain an exonuclease activity that removes thymine dimers from UV-irradiated DNA previously nicked with T4 UV endonuclease. This activity is not expressed if cells are infected in the presence of chloramphenicol. The enzyme has a requirement for divalent cation and is not affected by caffeine, but excision is inhibited in the presence of proflavine. The enzyme is present in all phage T4 mutants thus far examined, including 25 UV-sensitive mutants isolated during the course of the experiments, all of which are defective in the v gene. A similar activity can be detected in cells infected with phages T2, T3, and T6, but not in cells infected with phage T7.