Lethality and double-strand scissions from 14C decay in the DNA of micro-organisms

14C-2-thymidine was incorporated into the DNA of E. coli B/r and of coliphage T4. The labelled organisms were stored for several years at -1960C. Both were periodically assayed for loss of viability, and the coliphage also for the appearance of double-strand breaks (DSBs) in DNA, E. coli B/r exhibited a survival curve with a substantial initial shoulder, extrapolation number 5.2 +- 2.3, and a final exponential portion corresponding to a lethal efficiency per 14C decay per 2.5 x 109 daltons of DNA, of 0.009 +-0.002. For coliphage T4, our best estimate for the lethal efficiency per 14C decay is 0.03 +- 0.04, and that for the DNA breakage efficiency is -0.002 +- 0.004. The large standard errors result from the very small number of 14C decays occurring in each phage. These results suggest that 14C decay in the DNA of micro-organisms does not cause DSBs but does cause potentially lethal damage to the thymine bases in which decay occurs, and that wild-type E. coli can repair a large number of such DNA lesions. (author)