80 BACTERIOPHAGES 



direct and after effects is quite different from that caused by direct 

 effect or ultraviolet light. 



Buzzel and Lauffer (1952) studied the effect of X-rays on 

 phage T5 under conditions in which either the direct effect or the 

 indirect effect was predominant, and then investigated the sen- 

 sitivity to heat inactivation of the surviving phage population. 

 It was found that the survivors of the direct effect irradiation 

 have the same sensitivity to heat inactivation as an unirradiated 

 control population, while the survivors of the indirect effect ir- 

 radiation are far more heat-sensitive than the control. The 

 chemical agents responsible for the indirect effect thus increase 

 the heat-sensitivity of the survivors. The fact that the survivors 

 of the indirect effect are already modified in some way was also 

 shown by Watson (1952) who found that such survivors have an 

 increased X-ray sensitivity upon a second exposure to indirect 

 effects, i.e., that the indirect effect is cumulative (see also 

 Alper, 1954). In contrast, Watson found that direct damage is 

 not cumulative, i.e., that there is no evidence for "sublethal" 

 direct lesions. 



For purposes of reference, the available data for the sensi- 

 tivity of various phages to ionizing radiations are presented in 

 Table VI. 



4. Decay of Radiophosphorus 



It was discovered by Hershey, Kamen, Kennedy, and Gest 

 (1951) that T2 and T4 bacteriophages lose their infectivity upon 

 the decay (half-life of two weeks) of radiophosphorus P^^ atoms 

 incorporated into their DNA. The kinetics of this inactivation 

 proceed in such a manner that the surviving fraction of the 

 population is an exponential function of the proportion of P^- 

 atoms which have decayed by the time of assay. The rate of 

 inactivation is proportional to the specific radioactivity of the 

 medium in which the phage has been grown and is independent 

 of the phage concentration during decay, provided that the 

 particles are stored in sufficiently great dilution in a medium 

 which contains protective substances against any indirect effects 



