EFFECT OF RADIATIONS ON PHAGE PARTICLES 81 



produced by radiations attending the radioactive decay. These 

 observations indicate that the death of each radioactive phage 

 particle is the consequence of the disintegration of one of its own 

 atoms of P^-. It is possible to calculate the fraction of all radio- 

 active disintegrations which are actually lethal to the phage 

 particles in which they occur, on the basis of the inactivation rate 

 and the number of P^^ atoms per phage known to have disinte- 

 grated at various times. This fraction is approximately 0.1 in 

 the case of T2 and T4, an efficiency of killing so high that it is 

 possible to show by means of reconstruction experiments that the 

 ionizations produced on the way out of the virus particles by the 

 hard /3 electrons emitted at each P^^ disintegration cannot be the 

 principal cause of death. Instead, a short range consequence of 

 the radioactive disintegration, like the transmutation of phos- 

 phorus to sulphur or the recoil energy sustained by the decaying 

 nucleus, must be responsible for the loss of infectivity. When the 

 lethal effects of P^- decay were similarly studied in phages Tl, 

 T3, T5, T7, and lambda (Stent and Fuerst, 1955) and in the 

 salmonella phage P22 (Garen and Zinder, 1955), it was found 

 that the efficiency of killing per P^^ disintegration is very nearly 

 the same in all of these strains, i.e., that approximately one out of 

 every ten P^^ disintegrations kills any phage particle in which it 

 occurs. This efficiency depends on the temperature at which 

 radioactive decay is allowed to take place, rising from a minimum 

 value of 0.04 at -196° C. to 0.3 at +65° C. (the value of 0.1 

 holding only at +4° C.) (Stent and Fuerst, 1955; Castagnoh, 

 Donini, and Graziosi, 1955a, 1955b). On the basis of these 

 findings, Stent and Fuerst inferred that the efficiency of killing 

 reflects the double-stranded helical Watson-Crick structure of the 

 DNA which harbors the decaying P^^ atoms. They proposed 

 that the high proportion of nonlethal decays derives from the 

 possibility that the physiological function of the DNA macro- 

 molecules can be preserved even after radioactive decay has 

 interrupted one of the two single polynucleotide strands and that 

 the small proportion of lethal decays represent disintegrations 

 which have resulted in a complete cut of the DNA double helix. 



