DNA-DAMAGE IN BROMODEOXYURIDINE SUBSTITUTED SV40 DNA AND MINICHROMOSOMES FOLLOWING UVA IRRADIATION IN THE PRESENCE OF HOECHST-DYE-33258

Bromodeoxyuridine-substituted SV40 DNA and SV40 minichromosomes were p repared from infected CVI cells, and exposed to UVA light in the prese nce of different concentrations of Hoechst dye 33258. Following agaros e gel electrophoresis, the yields of ssbs and dsbs were determined. In DNA these yields are dependent on the level of BrdU substitution (0.0 38 ssb and 0.00022 dsb per BrdU residue/kJm(-2)), and on the mode of d ye binding (type I versus II). The yields of both ssbs and dsbs were f ound to be lower by factors of 1.25 and 2 respectively in minichromoso mes. UVA irradiation in the presence of the thiol cysteamine results i n lower strand break yields, presumably due to the repair of DNA radic als formed during photolysis. The thiol was found to react seven times more slowly with the DNA radicals than oxygen. The implications of th ese results are discussed in light of prior cellular studies.