Further studies of DNA damage and lethality from the decay of iodine-125 in bacteriophages

The DNA of coliphages T4 and T1 was labelled with 125I-iodode-oxyuridine. 125I decay is known to cause severe molecular damage via vacancy cascades (the Auger effect). We have compared the induction of both single- and double-strand breaks (SSBs and DSBs) in 125I-labelled T4 DNA stored at 1960C during the decay, either as intact phage or as free DNA. These comparative experiments indicated that, in addition to one DSB which apparently resulted directly from the Auger effect, each decay in an intact phage also gave rise to an additional 0.05 DSBs, as well as 1.6 SSBs, as a result of ionizing radiation absorbed in the same phage particles where the decay occurred. An examination of T4-killing by 125I decay revealed a two-phase survival curve, whose initial slope corresponded to a lethal efficiency per 125I decay of 0.95 +- 0.05, which was considerably higher than values previously determined. The results for phage T4, and of a more limited comparison of 125I suicide and DNA damage in phage T1, support the hypothesis that the vacancy cascades which accompany each 125I decay in DNA result in a double-strand break at the decay site and that each such break is a lethal event. (author)