Irradiation of circular DNA with 254 nm radiation or sensitization in the presence of Ag+ evidence for unwinding by photoproducts other than pyrimidine dimers

Structural alterations of DNA irradiated with UV light were analyzed. Relaxed, circular pAT 153 DNA molecules were sensitized by broad band radiation with a maximum at 313 nm in the presence of silver ions or irradiated with 254 nm light in buffer only. In both cases the electrophoretic mobility of DNA topoisomers was altered as a linear function of UV exposure. For DNA irradiated in the sensitized reaction the unwinding angle per site sensitive to Micrococcus luteus pyrimidine dimer endonuclease was found to be -11.40. The irradiated DNAs were a very good substrate for the Escherichia coli photoreactivating enzyme (PRE). However, the photoenzymic removal of all sites sensitive to the endonuclease specific for pyrimidine dimers was not coupled to a full restoration of the original electrophoretic mobility. Thirty and 23% of the unwinding were still present in the photoreactivated topoisomers and the unwinding angles per pyrimidine dimer were then recalculated as -10.10 and -8.70 for DNAs irradiated with 254 nm and sensitized, respectively. These results show that the tertiary structure of DNA is measurably altered by UV photodamages other than pyrimidine dimers. (author)