RADIOLYTIC SIGNATURE OF Z-DNA

Ionizing radiations induce various damages in DNA via the hydroxyl rad ical OH. generated by the radiolysis of water. We compare here the rad iosensitivity of BE and Z-DNA, by using a Z-prone stretch included in a plasmid. In the supercoiled plasmid, the stretch is in the Z-form, w hereas it is in the B-form when the plasmid is relaxed. Frank strand b reaks (FSB) and alkali-revealed breaks (ARE) were located and quantifi ed using sequencing gel electrophoresis. We show that B- and Z-DNA hav e the same mean sensitivity towards radiolytic attack, for both FSB an d ARE. Nevertheless, the guanine sites are more sensitive, and the cyt osine sites less sensitive in Z- than in B-DNA, leading to a character istic signature of the Z-form. The comparison of experiments with the outcome of a Monte Carlo simulation of OH. radical attack suggests tha t transfer of initial damage from a guanine base to its attached sugar or the adjacent 3' cytosine is more important in Z-DNA than in B-DNA.