Photochemical production of uracil quantified in bromodeoxyuridine-substituted SV40 DNA by uracil DNA glycosylase and a lysyl-tyrosyl-lysine tripeptide.

Exposure to UVA radiation of SV40 DNA substituted with bromodeoxyuridine (BrdU) in the presence of Hoechst dye 33258 results in the production of uracil. The yield of uracil was determined by measuring the increase in the single-strand break (SSB) yield after incubation of the photolyzed DNA with uracil-DNA glycosylase (UDG) in the presence of the tripeptide lysyl-tyrosyl-lysine (KYK). UDG removes uracil to leave an abasic site which is then cleaved to a SSB by KYK. The SSB yield was quantified by digital video imaging of ethidium fluorescence after separation of the I, II and III forms of SV40 DNA by agarose gel electrophoresis. Uracil is not detected when photolysis is carried out in the absence of the dye nor when unsubstituted DNA is used as the substrate. Without UDG or KYK treatment, the F0 for the loss of form I DNA is 100 J/m2. This falls to 13 J/m2 after incubation with UDG and KYK, indicating that uracil formation is approximately 5-fold more efficient than SSB formation. Formation of uracil suggests a mechanism for the high cellular toxicity of the dye-BrdU-UVA treatment.