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.
The average size of the dimer-containing acid-soluble oligonucleotides after incubation of specifically incised DNA with two dimer excising nuclease activities has been determined to be about 8 nucleotides. Kelly et al. (4) showed that the degradation of both the ultraviolet (UV)-irradiated homopolymer pair polydeoxythymine,00. polydeoxyade-nine4OOO [(dT)200-(dAj)000 I and randomly incised
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.