180 BACTERIOPHAGES 



becomes steeper, the sensitivity approaching again that of free 

 phage. This remarkable change in sensitivity to ultraviolet in- 

 activation during the first half of the latent period was com- 

 pletely unexpected and entirely different from the behavior of 

 phage T7. It cannot be explained on the basis of screening by 

 ultraviolet absorbing materials (Benzer, 1952). 



Latarjet (1948) studied the sensitivity of T2-infected bacteria 

 to inactivation by X-rays at various times during the latent pe- 

 riod. During the first 7 minutes, the sensitivity remained essen- 

 tially the same as that of extracellular virus. Then, during the 

 8th and 9th minutes there was an increase in resistance to X-rays 

 without a change in the multiplicity of the inactivation curves. 

 From the 9th to the 13th minute the multiplicity increased very 

 rapidly to a value of over 100 with little change in the sensitivity 

 to inactivation. For the remainder of the 21 -minute latent pe- 

 riod there was little change in the multiplicity of the inactivation 

 curves, but there was a gradual increase in X-ray sensitivity, 

 tending toward that of extracellular phage. 



An interpretation of the results obtained with T2 (Benzer, 

 1952) is that infection may be followed by a series of steps, each 

 step having a certain cross section for interference by ultraviolet 

 light. For free phage, or immediately after infection, the total 

 cross section is the sum of the individual cross sections. As de- 

 v^elopment proceeds, the completed steps drop out and the cross 

 section progressively decreases. This process could be related 

 in some way to multiplicity reactivation. Both the rapid change 

 in ultraviolet sensitivity during the latent period and the phenom- 

 enon of multiplicity reactivation occur with phages of the T2 

 serological group; neither efi'ect occurs in the T7 serological 

 group. 



The remarkable resistance of the phage-producing mechanism 

 to agents supposedly acting on the intrabacterial DNA can be 

 shown in another way. Phage particles containing P^^ of high 

 specific radioactivity are subject to "suicide" owing to the decay 

 of radiophosphorus, which presumably produces local damage in 

 the viral DNA. Stent (1955) applied this principle in experi- 



