PROCESS OF INFECTION AND VIRUS SYNTHESIS 



15 



their subsequent inactivatiou behavior is the same, and closely similar to the 

 sensitive behavior of Strain U2 (Siegel et al., 1957).^ 



10 



05 



0-2 



0-1 - 



005- 



002- 



■\ \ 



12 3 



UV dose (min) 



Fig. 4. Comparison of UV survival of infectious TMV nucleic acid irradiated in vitro 

 and irradiating leaf plus nucleic acid immediately after inoculation. From Siegel et al. 

 (1957). 



Key: O, Ul nucleic acid; •, Ul nucleic acid infective centers; Q, U2 nucleic acid; 



H, U2 nucleic acid infective centers; , U2; , U2 infective centers; . . . . , 



Ul and Ul infective centers. 



When a delay is introduced between the time of inoculation and time of 

 irradiation, a remarkable difference in the behavior of the nucleic acids, as 

 compared to intact virus, is noted, as shown in Fig. 5. Instead of undergoing 

 the phases noted for strain U2, or the long lag period for Ul before resistance 

 increases, the nucleic acids from both strains change their resistance to in- 

 activatiou by ultraviolet light almost immediately after making contact with 

 the host. Other findings (Siegel, unpublished) show that the exponential 

 curve changes to a multitarget curve m a matter of only 3 hours after infec- 

 tion commences, in the case of the nucleic acids, as compared to the 7 hours 

 or more for such a change to occur with the intact virus strains. 



Thus, it seems clear that infectious nucleic acid can start reproducing more 

 rapidly than intact virus, and we can feel some confidence in ascribing the 

 first three phases m changing behavior of mtact virus as associated with the 

 freeing of the nucleic acid from the protein subunits as a preliminary step in 

 the redupHcation of the nucleic acid. Obviously, it would be of the greatest 



* McLaren and Takahashi (1957) have also performed in vitro radiation inactivation 

 experiments -w-itli TMV and TMV infectious nucleic acid. Where direct comparison ia 

 possible, there is a large measure of agreement on the experimental findings. 



