Adjacent guanines as preferred sites for strand breaks in plasmid DNA irradiated with 193 nm and 248 nm UV laser light

Abstract

193 nm UV laser light induces single strand breaks as well as double strand breaks in plasmid DNA. The frequency of strand breaks is increased at sites where at least two guanines or, less frequently, a guanine and an adenine are adjacent to each other. 248 nm UV laser light induces predominantly single strand breaks with a less pronounced preference for guanine clusters. At both wavelengths, the presence of oxygen does not change the pattern of strand breaks, but in the presence of nitrous oxide, selectivity is lost; this is attributable to the production of the hydroxyl radical. These findings can be explained by a model in which the propagation of a radical or an electron hole along the DNA helix competes kinetically with the strand cleavage reaction. The difference in selectivity at the two different wavelengths is ascribed to the preferential light absorption by the purine bases at 193 nm.