G. S. STENT AND C. R. FUERST 453 



that no photoreactivation of the P-^^-inactivated T2 took place, although the 

 titer of the ultraviolet-inactivated control was raised by nearly a factor of 100 

 by exposure to visible light. 



DISCUSSION 



Cause of Death. — 



An atom of P^^ decays into the stable isotope of sulfur, S^^, upon ejection 

 of a beta electron-neutrino pair of total kinetic energy 1.7 mev. The beta 

 particle produces ionizations along its path, which are capable of damaging 

 biological materials in a way similar to x-rays. Hershey, Kamen, Kennedy, 

 and Gest, however, showed by means of calculations based on the volume of 

 the T2 particle, the density of ionizations along the beta track and the known 

 efficiency of killing per x-ray ionization, or by reconstruction experiments 

 in which non-radioactive phage particles were irradiated with beta particles 

 emitted by external, non-incorporated P^^ atoms, that beta particle ioniza- 

 tions could not be the principal cause of the inactivation of radioactive bacteri- 

 ophage particles. Hershey et al. concluded, rather, that a short range conse- 

 quence of the nuclear reaction, e.g. the recoil sustained by the disintegrating 

 nucleus upon ejection of beta electron and neutrino, or the transmutation 

 of phosphorus into sulfur, was responsible for death. The present finding that 

 the sensitivity of radioactive phages to P'^ decay is reduced only slightly 

 after infection supports this view. For, it appears likely that the state of ag- 

 gregation of the phage DNA is more compact in the phage head than in the 

 protoplasm of the host cell (Watanabe, Stent, and Schachman, 1954). Hence 

 the chance of irradiation of one part of the phage DNA by distant P^^ atoms of 

 another would have been seriously reduced once infection was under way. 



Efficiency of Killing. — 



Hershey et al. suggested that the fact that only one P^^ disintegration in 

 about ten was lethal to T2 or T4 might reflect a division of the phage DNA 

 into 10 per cent "essential" and 90 per cent "non-essential" structures. Under 

 this view, any P^^ disintegration in the former would be surely lethal and any 

 in the latter generally harmless. The present finding that a is nearly the same 

 in various phage strains of greatly different size, morphology, and biological 

 properties makes this hypothesis less likely. The dependence of a on tem- 

 perature, furthermore, excludes the possibility that the anatomy of the phage 

 is the sole factor responsible for the efficiency of killing. It seems, rather, 

 that a must at least in part reflect some structural aspect of the DNA mole- 

 cule, the substance whose function is presumably destroyed by the decay of 

 its radioactive P^^ atoms. 



The lethal effects of P^^ decay can perhaps be best understood in terms of 

 the macromolecular structure of DNA, recently uncovered by Watson and 



292 



