Lethality and DNA Breakage from ³²P and ³³P Decay in Bacteriophage T4

Summary

To examine the importance of nuclear recoil in biological damage from ³²P decay, we have compared the production of double-strand breaks (DSB) in the DNA of coliphage T4 by ³²P and ³³P. ³³P has the same decay scheme as ³²P, but a much lower nuclear recoil energy (1·7 eV versus 20 eV). When labelled phage are stored at − 196°C in a glycerol-casamino-acid storage medium, the lethal efficiency per decay is 0·058 for ³²P and 0·022 for ³³P. The efficiency of induction of DSB's determined by neutral sedimentation of extracted DNA, is 0·057 for ³²P and only 0·014 for ³³P. These results indicate that the great bulk of ³²P lethalities in this phage at − 196°C are due to DSB's resulting from nuclear recoil. Killing by either isotope is increased when storage is at 4°C; and the presence of 2-aminoethylisothiouronium (AET) gives partial protection against killing at 4°C. However, the induction of DSB's by ³²P decay is little affected by these variables or by storage as extracted DNA versus intact phage, all as predicted if DSB's result primarily from nuclear recoil.