RADIATION AND VIRUSES 351 



thetic machinery of the host, interference is probably also a manifestation 

 of the appropriation by one virus of the directive pattern of specificity to 

 the exclusion of another virus. It is interesting in this connection that 

 "lysogenic" bacteria, which carry a phage in a form that does not inter- 

 fere with bacterial life, can be infected and lysed by other, unrelated 

 phages and can liberate them normally. 



In the case of animal and plant viruses, interference phenomena have 

 generally been studied only in their mass manifestations when a tissue or 

 a whole organism is exposed to two viruses in succession or simultaneously. 

 It is difficult therefore to interpret interference in terms of cellular events. 

 For animal viruses, interference phenomena have been discussed by Henle 

 (1950) and Schlesinger (1950b). Interference may occur between related 

 or unrelated viruses, but not all unrelated viruses interfere with one 

 another, and two viruses can often be shown to multiply in the same cell. 

 Whenever there is interference between two active viruses, it is also 

 observed with virus . ^activated by ultraviolet radiation ; other radiations 

 have hardly been studied in this connection. With influenza viruses in 

 the allantoic cavity of the chick embryo, it has been shown (Henle and 

 Henle, 1943, 1945; Ziegler et al., 1944) that a large amount of a virus, e.g., 

 influenza type A, after inactivation by ultra\iolet radiation can prevent 

 reproduction either of homologous or of heterologous active virus, e.g., 

 influenza type B. This interference was at first attributed to suppression 

 of virus adsorption because the irradiated virus destroyed the virus recep- 

 tors on the allantoic cells, 't is now known, however, that interference 

 may occur with amounts of virus that do not prevent adsorption and also 

 by introduction of the interfering virus after the first virus has been 

 adsorbed (Henle, 1950). In the case of bacteriophage, as well as influenza 

 virus, interference probably takes place at the level of the reproductive 

 process. That a blockade of the synthv^^c machinery is involved, rather 

 than a competition for building blocks, is suggested by the fact the 

 inactive virus, although unable tc reproduce, retains the interfering ability. 



4-2. REACTFx vTION PHENOME A 



Reactivation phenomena hi. oeen reported only with bacteriophages, 

 but the possibility of their occurrence in other viruses should be explored. 

 Bacteriophages exposed to radiation give different activity titers accord- 

 ing to the conditions of titration. Two factors have been found relevant : 

 The number of irradiated particles absorbed per bacterium (Luria, 1947) 

 and the exposure of the infected bacteria to light of certain wave lengths 

 (Dulbecco, 1950). Xo reactivation effect has been observed following 

 treatments of irradiated phage before adsorption to the host bacterium. 

 Reactivation of phages by light ("photoreactivation") is discussed in 

 detail in Chap. 12 of this volume. 



4-2a. MuUiplirify Reactivation. In phage titration the phage must be 



