Lethality and Double-strand Scissions from ¹⁴C Decay in the DNA of Micro-organisms

Summary

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°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 × 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.