350 UADIATION lUOLOGY 



ap|)iv('iahk' amount of multiplicity reactivation was found' with phage 

 inactivated by the aftcretTect of X rays (see Sect. 2-1 a). 



6. INTRACELLULAR IRRADIATION OF VIRUSES 



5-1. IHHADIATION OF CELL8 INFECTED WITH EXOGENOUS VIRUSES 



Irradiation of cells durinjj; infection with \iruses may be of use in the 

 study of \irus reproduction. This approach has as yet been limited to 

 bacteriophage, but it could be applied to other viruses, particularly in 

 tissue cultures. With bacteriophage the basic experimental procedure 

 (Anderson, 1944; Luria and Latarjet, 1947) consists in infecting a bacterial 

 culture with virus, taking samples at intervals during the period that pre- 

 cedes lysis, and exposing them rapidly to various doses of ratliation. The 

 irradiated infected bacteria are then tested immediately for their ability to 

 liberate phage. This ability can be suppressed by either ultraviolet or 

 X rays; and, if the fraction of bacteria that liberate phage is plotted 

 versus dose of radiation, "suppression curves" are obtained. The sup- 

 pression effect is exerted on the intracellular bacteriophage rather than on 

 the bacterial host. This is shown by the following observations: 



1. Active phage can reproduce normally in bacteria exposed to ultra- 

 violet radiation shortly before infection (Anderson, 1948). 



2. If infected bacteria are irradiated immediately after infection, the 

 rate of suppression of phage liberation as a function of radiation dose is 

 similar to the rate of inactivation of free virus. 



3. In multiple infection the suppression cur\e immediately after infec- 

 tion is of the multiple-hit type and closely resembles the curves obtained 

 for active phage production in multiple infection with irradiated bacterio- 

 phage (see Sect. 4-2a). 



As the time after infection increases, the suppression curves change in a 

 manner characteristic for the phage. The simplest case is that of phage 

 T7 (Benzer, 1952; see Fig. 9-2). After infection there is no change in 

 ultraviolet sensitivity for 3 or 4 min, but then the suppression curve 

 becomes of the multiple-hit type without any change in the final slope of 

 the curve; this seems to indicate a simple mechanism of multiplication of 

 virus elements having the same sensitivity as the free virus. 



With phage T2 (Luria and Latarjet, 1947; Benzer, 1952) the first 

 change in bacteria infected with one T2 particle and exposed to ultra- 

 violet (2537 A) is an increase in ultraviolet resistance without appreciable 

 change in the shape of the curve. Several minutes later the inactivation 

 curve changes to a complex type, suggesting an effect on numerous 

 objects within each cell. In the latest stages of infection, radiation 

 sensitivity again increases. The results suggest that phage T2 must 

 perform an early function easily blocked by ultraviolet damage and that 

 the radiation sensitivity increases as this early phase is passed. 



