362 BACTERIOPHAGES 



b. Multiplicity Reactivation of Phage Inactivated by Ionizing 

 Radiations or by P^^ Decay 



A certain difficulty to explain multiplicity reactivation as a 

 recombination phenomenon arose from the fact that the extent 

 of multiplicity reactivation was found to be strongly dependent 

 on the inactivating agent. Watson (1950) reported very slight 

 multiplicity reactivation after hard X-ray inactivation, and 

 multiplicity reactivation of P^^.jj-^activated phage was not ob- 

 served at all (Stent and Fuerst, 1955). However, Weigle and 

 Bertani (1956) demonstrated that X-ray inactivated T2 under- 

 goes multiplicity reactivation with high efficiency if the radia- 

 tion is applied after the phage has been adsorbed to the cell. 

 They concluded that, besides the usual lethal damage. X-rays 

 cause an additional "early step damage" which is expressed 

 only when occurring before adsorption. The fact that under 

 suitable conditions X-ray inactivations may be multiplicity 

 reactivated to a similar extent as ultraviolet inactivations, 

 although the two kinds of radiation are supposed to act in 

 very different ways, argues strongly against a direct chemical 

 reversal of the damage in multiplicity reactivation. The failure 

 to observe multiplicity reactivation following damage by de- 

 cay of P^2 remains unexplained. 



c. Cross- Reactivation 



The progeny of mixed infections of bacteria with ultraviolet- 

 inactivated phage and genetically marked viable phage con- 

 tains genetic markers of the inactivated parent (Luria, 1947). 

 This has been called cross-reactivation, but should perhaps 

 more properly be referred to as "marker rescue," because the 

 inactivated phage is not reactivated as a whole. Both the 

 probability of marker rescue and the partial burst size of phage 

 carrying a given rescued marker decrease with increasing dose 

 of radiation. Extensive analysis of cross reactivation in T4 

 (Doermann, Chase, and Stahl, 1955; Krieg, 1957) leads to the 

 following conclusions concerning the nature of the radiation 



