Induction of Lethality and DNA Breakage by the Decay of Iodine-125 in Bacteriophage T4

SummaryIodine-125 has been incorporated into the DNA of coliphage T4 in the form of 5-iododeoxyuridine (IUdR), an analogue of thymidine. 125I decays by electron capture with consequent vacancy cascades and multiple ionization (Auger effect). This decay mode is known to cause extensive fragmentation of labelled small molecules. 125I-labelled T4 phage have been stored in a glycerol-casamino acid storage medium at −196°C during decay and assayed for the induction of lethalities and double-strand breaks (DSB's) in DNA. The lethal efficiency per decay, α(lethal), is 0·59 ± 0·08, in reasonable agreement with the previously determined value for 125I decay in T1 phage, α(lethal) = 0·49 ± 0·03. The efficiency of induction of DSB's, as determined by sedimentation of extracted DNA in neutral sucrose gradients, is 1·01 ± 0·13. These results indicate that every decay causes a DSB, but that some DSB's are non-lethal. α(DSB) is independent of storage temperature during decay (+4°C versus −196°C) and is also independent of whether storage is as intact phage or as free DNA. This suggests that the observed DNA breakage is due to local 125I decay effects, rather than to the ionizing radiation emitted during decay.