INFLUENCE OF DNA CONFORMATION ON RADIATION-INDUCED SINGLE-STRAND BREAKS

It is usually assumed that sparsely ionizing radiation produces random ly distributed DNA breakages. This seems to be supported by the findin g that in some DNA fragments single-strand scissions occur uniformly a t all nucleotide sites, regardless of sequence. We performed experimen ts on two DNA fragments of about 300 bp having different conformation to test whether radiation-induced single-strand breakage is dependent on DNA conformation. Breakage analysis was carried out by denaturing p olyacrylamide gel electrophoresis, which allows determination of the b roken site at single nucleotide resolution. We found uniform cutting p atterns in B-form regions. On the contrary, X- or gamma-irradiation of curved fragments of kinetoplast DNA showed that the distribution of s ingle-strand breaks was not uniform along the fragment, as the cleavag e pattern was modulated in phase with the runs of A-T pairs. This modu lation likely reflected the reduced accessibility of the sites which o n hydroxyl-radical attack give rise to strand breaks. The cleavage pat tern was phased with the runs of A-T pairs. Moreover, the overall yiel d of strand breaks was considerably lower in curved DNA fragments than in those with extended straight regions. The conformation effect foun d here indicates that the cleavage pattern reflects the fine structura l features of DNA.