INACTIVATION OF VIRUSES 381 



shown that inactivation through P^^ decay, like UV inactivation, is a result 

 of damage to the nucleic acid portion of the virus particles. 



The ejected electron has a mean energy of 0-7 Mev and behaves like the 

 electrons produced by hard X-rays or y-rays; i.e., it produces primary ioniza- 

 tions which, to begin with, are several thousand A apart. The mean energy of 

 the recoiling nucleus, of about 20 ev, may cause disruption of the polynucleo- 

 tide chain in which the P^^ atom formed a link, but, in most cases, the recoil 

 effect would seem to remain confined to one cham (Stent and Fuerst, 1955). 



The first experiments of this type were carried out with the phage T2 by 

 Hershey et at. (1951), The following important conclusions could be drawn: 



(1) the instability of labeled phage particles is due to the nuclear transmuta- 

 tion and/or the recoil effect; only a smaU percentage of the total effect is 

 accounted for by ionizations, very few of which occur in the virus particle; 



(2) inactivation is strictly exponential, with a rate proportional to the specific 

 activity; and (3) only one out of about 10 P^^ disintegrations causes inactivation. 



These findings could mean that not more than about 10 % of the DNA of 

 phage T2 is essential for reproduction and that a single decay process occur- 

 ring in this part causes inactivation. This target type interpretation is ruled 

 out, however, because it has been shown that inactivation depends on tem- 

 perature. Stent (1953) and Stent and Fuerst (1955) determined the rates of 

 inactivation of P^^-labeled phages at 4° and at — 196°C., respectively. For 

 the coliphages Tl, T2, T3, T5, and T7, and for phage A they consistently 

 found a ratio of about 0-6 between the inactivation rates observed at — 196° 

 and 4°C., respectively. The constancy of the temperature effect suggests that 

 the mechanism of inactivation is the same for aU the phages studied. It is 

 perhaps significant that a temperature effect of the same magnitude was fomid 

 for inactivation of phage Tl by X-rays (Bachofer et al., 1953; see also p. 367). 



In agreement with these experiments, Castagnoli and Graziosi (1954) have 

 shown that a megatherium phage labeled with P^- was inactivated more 

 slowly at — 79°C than at 4°C. 



Garen and Zinder (1955) studied the phage P22, derived from Salmo7iella 

 typhimurium. The transducing capacity of this phage was much more resis- 

 tant to the inactivating effects of both UV and incorporated P^^ than were 

 the lytic and lysogenic activities. The fraction of the sensitive material 

 (DNA) engaged in transduction would thus seem to be small compared to 

 that which is necessary for reproduction or lysogenization. The relative sensi- 

 tivity to X-rays and to UV was found to differ for several properties of the 

 phages P22 and T2; in all instances inactivation due to incorporated P^^ gave 

 results similar to those obtained with X-ray inactivation. These experiments 

 support the idea that the damage caused by decay of incorporated P^^ is 

 similar to that produced by X-rays, and that it resembles the effect of UV 

 only insofar as both are confined to the nucleic acid structures of the virus. 



