PROCESS OF INFECTION AND VIRUS SYNTHESIS 



11 



light wlien the virus is irradiated in a thin, liquid layer, and then rubbed on 

 leaves to test its remauiing uifectivity. With strains Ul and U2, the fraction 

 of virus particles surviving the radiation treatment is an exponential func- 

 tion of the dose of apphed radiation. Such "single-hit" inactivation curves 

 for the common strain of TMV had been found previously by Price and 

 Gowen (1937), Oster and McLaren (1950), and Bawden and Kleczkowski 

 (1953). Only the slopes of the straight line inactivation curves are different, 

 as shown by the data plotted in Fig. 2. TMV strain U2 is much more sensitive 

 to inactivation by radiation than is strain Ul.^ 



Fig. 2. A comparison of the survival curves obtained when TMV virus suspensions 

 are irradiated in vitro with UV, and when leaves plus virus are irradiated soon after 

 inoculation. Triangles represent fraction of virus surviving when irradiated m vitro. 

 Circles represent number of lesions appearing on irradiated leaves plus virus. From 

 Siegel and Wildman (1956). 



Figure 2 also illustrates that nearly the same difference in sensitivity to 

 inactivating radiation by strains Ul and U2 is maintained after virus has 

 made contact with the host cells, for the same type of inactivation curves 

 were obtained when the leaf was first rubbed with virus, and then the leaf 

 and attached \'irus irradiated together. There is a small increase in slope for 

 the U2 strain which remains unexplained, but the experimental points for 

 the Ul strain can be fitted to the same inactivation curve, irrespective of 



^ An explanation for the difference in sensitivity between the Ul and U2 strains of 

 TMV has been found by Siegel (1957) to reside in a difference in the bonding between 

 the nucleic acid and the protein subimits of the virus particle. 



