RADIOBIOLOGY OF BACTERIOPHAGE 379 



IV. X-Eay Inactivation of T-Even Phage 



A. Survival of Infectivity 



X-rays applied either intracellularly or extracellularly produce exponen- 

 tial inactivation (Fig. 1) under conditions in which indirect effects are 

 avoided (see Chapter V, Volume I). Whereas with both ultraviolet light and 

 suicide the phage immediately following infection is slightly less sensitive 

 than free phage (Benzer, 1952; Stent, 1955), with gamma rays the sensitivity 

 is the same in both cases (Harm, 1958a), and with soft X-rays, the infected 

 cells are more sensitive than the free phage (Harm, 1958a; Symonds and 

 McCloy, 1958). The equahty in sensitivity with gamma rays indicates that 

 essentially all of the damage produced by ionizing radiations is damage to 

 the DNA of the phage, and that the biological effectiveness of an individual 

 ionization does not change as a consequence of injection. The increase in 

 sensitivity to soft X-rays following injection of the DNA must, then, reflect 

 a relatively "inefficient" distribution of hits among phages irradiated as free 

 particles with soft X-rays. This inefficiency results from the occasional 

 production by one photoelectron of two or more effective ionizations within 

 the same particle. (The change to a more efficient distribution upon in- 

 jection reflects a change in shape of the DNA mass of the phage from a 

 compact form to an elongated or flattened form (Harm, 1958a).) It follows 

 that at any given survival level, the average number of hits per inactive 

 particle among free phages irradiated with soft X-rays is greater than is 

 the case with gamma rays or with phage irradiated after injection. This 

 may in part account for the differences in behavior (see below) of phage 

 inactivated with soft or hard X-rays, or with soft X-rays applied before or 

 after infection. 



B. Multiplicity and Cross Reactivation 



For radiation apphed to free phage, the degree of multiplicity reactivation 

 is shght and increases with decreasing ionization density (Weigle and 

 Bertani, 1956). The dependence on ionization density is probably a simple 

 consequence of the bunching of hits described above (Harm, 1958a). 



When X-rays are appHed to phage-infected ceUs, multiplicity reactivation 

 occurs with an efficiency comparable to that found with ultraviolet light 

 (Weigle and Bertani, 1956; Harm, 1958a). Cross reactivation, which is 

 rather inefficient when X-rays are apphed to free phage (Doermaim and 

 Chase, reported by Stahl, 1956; Harm, 1958a) is also higlily efficient when 

 the radiation is apphed to cells followuig infection (Harm, 1958a). It seems 

 likely that when ionizing radiation is applied to free phage a dose-dependent 



