| Abstract: | Abstract— DNA from Escherichia coli was irradiated at 254 nm in the presence of silver in order to preferentially enhance the rate of formation of pyrimidine-dimer damage over nondimer damage. The irradiated DNA was treated with formaldehyde in order to measure the unwinding velocity of the defects associated with the pyrimidine dimers. This velocity was found to be 0.18 base pairs/min per pyrimidine dimer, which is nearly 8 times less than that found for a double-strand break (1.37 base pairs/min) obtained by use of sheared DNA whose size was determined by electron microscopy. The rate of reaction of the DNA with formaldehyde varied linearly with the pyrimidine dimer concentration and showed no inflection due to clustering. Treatment of irradiated DNA with UV endonuclease enhanced the formaldehyde reaction by ? 7-fold, consistent with the conversion of a dimer into the faster-reacting defect associated with a single-strand break. These results indicate that the distribution of dimers in DNA is random and not clustered, and that previous interpretations of clustering were based on the false assumption that dimer and chain break defects unwind with similar velocities when treated with formaldehyde. |
