Gamma-ray-induced Thymine Damage in the DNA in Coliphage ϕX174 and in E. Coli

Summary

The radiation chemical reactivity of thymine residues has been compared for single-stranded ϕX174 DNA free in solution and in the intact bacteriophage, as well as for native E. coli DNA in solution and inside the cell. The γ-ray-induced release of [³H]H₂O from thymine-methyl[³H] was measured to assess the reactivity of the thymine methyl group. Formation of ring-saturated products of the 5,6-dihydroxy-dihydrothymine type (t′) was determined by an alkali-acid degradation assay. It was observed that the formation of [³H]H₂O was suppressed by a factor of 9 in intact ϕX174 bacteriophage compared to ϕX174 DNA, whereas the formation of t′ was only slightly suppressed in the phage. For E. coli cells the formation of [³H]H₂O was reduced 790-fold compared with free E. coli DNA, and t′ formation was reduced 140-fold.

The extents of survival after γ-irradiation of ϕX174 phage plaque-forming ability and E. coli colony-forming ability were also determined. Under the conditions used in these experiments, a linear relationship was observed between the loss of biological activity and the formation of thymine damage for both ϕX174 phage and E. coli cells.