DNA Strand Breakage by ¹²⁵I-Decay in a Synthetic Oligodeoxynucleotide: Quantitative analysis of fragment distribution

Abstract

The DNA breakage produced by decay of ¹²⁵I in a double-stranded 41 bp oligodeoxynucleotide was investigated by DNA sequencing gel analysis. Use of both 5'- and 3'-end ³²P labelling of the ¹²⁵I containing strand provided the single-stranded breakage pattern at either side of the ¹²⁵I-dC. The asymmetric pattern of breakage relative to the ¹²⁵I-labelled nucleotide enabled deconvolution of two components of breakage. One of them declines very quickly with nucleotide number from ¹²⁵I-dC and dominates within 4-5 nucleotides. We assume this component to be associated with neutralisation of charged Te atom resulted from decay, or/and with radiation damage to an initial target other than the deoxyribosyl moiety-probably the bases. The second component depends on the geometrical distance between the ¹²⁵I atom and deoxyribosyl atoms. These two components are responsible for breakage under conditions limiting radical mediated damage, namely in the presence of 2M dimethylsulphoxide. The third component, associated with radical-mediated damage, contributes to the total breakage during incubation in 20mM phosphate buffer alone, and under these incubation conditions dominates the breakage beyond 8-9 nucleotides from ¹²⁵I-dC. The estimated average numbers of single-stranded breaks produced in the ¹²⁵I-containing strand by each mechanism in 41-mer oligodeoxynucleotide with ¹²⁵I-dC at 21st position are respectively 2.33, 0.98 and 0.63.