Mechanism of repair of DNA in bacteriophage. I. Excision of pyrimidine dimers from ultraviolet-irradiated DNA by an extract of T4-infected cells.

Abstract

Abstract When [14C]thymine-labeled T4 DNA, after exposure to ultraviolet light, was incubated with an extract of T4-infected Escherichia coli, pyrimidine dimers were selectively released into the acid-soluble fraction; the rate of release of dimers was about five times greater than that for thymine. The reaction requires Mg2+ and proceeds at pH 7 to 8 in the dark. The amount of thymine and thymine-containing dimers released from the ultraviolet-irradiated DNA into the acid-soluble fraction was determined by Dowex 1 column chromatography. Dimers lost from the DNA were almost quantitatively recovered, in the form of nuoleotides but not of free bases or nucleosides, in the acid-soluble fraction. As a result of the specific release of dimers, the content of dimers in the DNA decreases progressively during the incubation. Thus the excision in vitro is similar to the reaction observed in vivo in ultraviolet-resistant cells. An extract of normal cells does not exhibit the preferential release of dimers even when the homologous E. coli DNA is used as substrate. Only a small amount of dimers is released into the acid-soluble fraction during the incubation of irradiated E. coli or T4 DNA with the normal cell extract. Thus the level of dimer-excising activity of normal cells, if any, must be very low compared with that of T4-infected cells. It is likely that enzyme(s) responsible for excision of dimers is induced by infection with T4.