Sequence-dependent variations of DNA structure modulate radiation-induced strand breakage.

Using a 80 base pair DNA fragment, the sequence-dependence was compared for: (i) the probability of fast neutrons induced strand breakage, (ii) the accessibility of the H4'- and H5'-atoms to OH. attack, (iii) the width of the minor groove, and (iv) the probability of OH. reactions with H4'- or H5'-atoms. The probability of strand breakage was measured using sequencing gel electrophoresis. The accessibility and the probability of reaction were calculated for the energy-minimized modelled DNA fragment. A Monte-Carlo simulation was used for calculating the probabilities of H-atom abstraction by OH.. It was observed that reduced breakage occurs in sequences exhibiting low accessibility of H4' and H5'2 and low probability of H-atom abstraction by OH., due to a narrow, minor groove. This shows that the breakage probability at a given nucleotide site is not determined by the chemical nature of the nucleotide (A, T, G or C), but mainly by the local sequence-modulated intrinsic structure. Fitting the experimental results with the calculated probabilities of reaction suggests that a C4'-centered radical evolves towards a strand break three times more efficiently than the C5' one, and that half of the breaks occur via the 4'-path and half via the 5'-path.