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 -196 degrees C. 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 10(9) 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.