284 



HEINZ SCHUSTER 



1.0 p 



0.5 



0.2 - 



0.1- 



Hours After Inoculation 



Fig. 10. Ultraviolet light survival of infective centers from TMV strains and 

 nucleic acids derived therefrom. Dose of UV: intact U2 and RNA, 90 seconds; Ul, 5 

 minutes. # = Ul RNA, □ = U2 RNA, V = intact U2 , O and X = intact Ul . 

 NA = RNA. [A. Siegel, W. Ginoza, and S. G. Wildman, Virology 3, 554 (1957).] 



TMV and virus RNA from two strains was studied by Siegel et al. V2i - 125 

 It was possible to differentiate several phases of the infection. Intact virus 

 from strains Ul or U2 was inoculated on leaves and the infective centers 

 irradiated with a constant dose of UV light at different times after infection. 

 For some time the UV sensitivity of the centers remained unchanged 

 (phase 1). This "lag phase" was 5 hours long for strain Ul and 2.5 hours 

 long for strain U2. This was followed by an increase in the resistance of the 

 infective centers (phase 2), before virus replication begins. On the other 

 hand, if infectious RNA from either strain was inoculated and the resulting 

 centers irradiated, the resistance of the centers increased quite soon after 

 infection (Fig. 10). This difference in sensitivity to UV light when infection 

 is initiated with intact virus or with RNA suggests that the initial events of 

 TMV infection are concerned with the release of RNA from the protein 

 moiety of the virus particle. The locality of this release would be close to 

 the surface of the leaf since the resistance to UV during the lag period is 

 little changed from that of the extracellular virus particle. One might ex- 

 plain the longer lag phase of strain Ul by assuming that RNA is released 

 with greater difficulty from Ul particles. This suggested greater affinity 



124 A. Siegel and S. G. Wildman, Virology 2, 69 (1956). 



125 A. Siegel, W. Ginoza, and S. G. Wildman, Federation Proc. 16, 248 (1957) ; Virology 

 3, 554 (1957). 



