346 i; \i)i A ri»).\ HioLOOY 



sion of any one of the steps needed for sueeossful infection and virus repro- 

 duction, it is often more easily affected than any other recof^ni/ahle prop- 

 erty of the virus particles. Changes produced by radiation, other than 

 inactivation, may he even more interesting than inactivation itself since 

 they may reveal new properties of the virus particles and their dependence 

 on the integrity of specific virus functions. 



Only very large radiation doses cause actual disintegration of the par- 

 ticles. In this section are considered, first, nonlethal efTects of radiation, 

 i.e., changes recognizable in virus particles that survive irradiation; then a 

 series of changes recognizable^ in iiiacti\-e particles. 



3-1. NONLETHAL EFFECTS 



Nonlethal effects are recognized as alterations in the properties of those 

 virus particles that survive exposure to radiation; some alterations are 

 nonhereditary, others are transmitted to the progeny. Among the former 

 may be mentioned a delay in reproduction of bacterioptiage particles that 

 survive ultraviolet irradiation, as evidenced by an increase in the latent 

 period between infection of a bacterium and its lysis with liberation of new 

 virus (Luria, 1944). The new virus gives a normal growth cycle; the 

 reproductive delay, then, persists for only one cycle of intracellular repro- 

 duction. Another nonlethal effect consists in a slower adsorption by 

 bacteria of phage surviving exposure to X rays under conditions where 

 indirect effects are prevalent (Watson, 1952) ; there is probably a surface 

 alteration of the phage by toxic substances produced by X rays in the 

 medium. 



A more important group of nonlethal effects of radiation on viruses is 

 the induction of phenotypic mutations, a field as yet insufficiently investi- 

 gated. Exposure of tobacco mosaic virus to X rays has been reported to 

 produce mutations both from wild type to aucuba and back ((lowen, 

 1941). The data indicate that the probability of inducing a mutation is 

 about one one-thousandth that of inactivating a virus particle. A report 

 is available on mutation induced in tobacco mosaic virus by irradiation of 

 virus-infected leaves (Pfankuch el al., 1940). 



With bacteriophage T2, Latarjet (1949) has reported that, following 

 ultraviolet irradiation of infected bacteria, there is an increase in the pro- 

 portion of bacteria that liberate phage mutants T2h. 



3-2. SEPARATION OF PROPERTIES OF INACTIVE VIRUS PARTICLES 



The detection of the effect of radiations on different properties of viruses 

 depends on the number of properties recognizable by the limited mode of 

 analysis. With viruses such as bacteriophages and influenza viruses, 

 several properties can be separated by increasing doses of radiation or by 

 different types of radiations. Some groups of properties, however, are 

 always lost simultaneously. When the properties studied represent 



