Search for a mechanism for the increased sensitivity of 5-bromouracil-substituted DNA to ultraviolet radiation

Abstract

1. 1. The 5-bromouracil-substituted DNA's of Escherichia coli 15T − and T3 coliphage were labeled in thymine and 5-bromouracil with different isotopes and exposed to 280 nm radiation. Acid and enzymic hydrolysates of the substituted DNA's of the above organisms were analyzed for photoproducts by paper chromatography. 2. 2. The same four or five photoproducts, derived from 5-bromouracil, were detected in the acid hydrolysate of the irradiated DNA's under all conditions tested, including irradiation in vivo and in vitro and in the presence and absence of cysteamine. 3. 3. No product containing both the 5-bromouracil and thymine labels could be detected. 4. 4. Quantitative differences in the distribution of photoproducts were obtained under the various conditions of irradiation. Cysteamine, when present during irradiation, caused a decrease in the formation of uracil but had little effect on the total photodecomposition of 5-bromouracil. Also, an increase in dose caused an increase in the contribution of uracil to the total photoproducts. 5. 5. At low doses there is relatively little difference between the photosensitivity of 5-bromouracil and thymine in DNA. 6. 6. Exposure in solution to 254 nm radiation only slightly affects the chromatographic mobility of the photoproducts derived from 5-bromouracil. 7. 7. The excision in vivo of thymine-containing dimers from the irradiated DNA is unaffected by the presence of 5-bromouracil in the DNA. 8. 8. The relatively smaller numbers of thymine-containing dimers in irradiated DNA substituted with 5-bromouracil are quantitatively explicable in terms of the random substitution of bromouracil for thymine. 9. 9. The implication of these data is discussed with reference to the suggested mechanisms of the radiation sensitization of DNA by 5-bromouracil substitution.