Gamma-ray-induced thymine damage in the DNA in coliphage phi chi 174 and in E. coli.

The radiation chemical reactivity of thymine residues has been compared for single-stranded phi chi 174 DNA free in solution and in the intact bacteriophage, as well as for native E. coli DNA in solution and inside the cell. The gamma-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 (see article) was determined by an alkali-acid degradation assay. It was observed that the formation of that the formation of [3H]H2O was suppressed by a factor of 9 in intact phi chi 174 bacteriophage compared to phi chi 174 DNA, whereas the formation of (see article) was only slightly suppressed in the phage. For E. coli cells the formation of [3H]H29 was reduced 790-fold compared with free E. coli DNA, and (see article) formation was reduced 140-fold. The extents of survival after gamma-irradiation of phi chi 174 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 phi chi 174 phage and E. coli cells.