14 S. G. WILDMAN 



have some information which indicates that a much longer period of time 

 ensues after infection with Ul before a detectable rise in resistance to radia- 

 tion is apparent. Whereas a change in slope of the U2 inactivation curve is 

 apparent within 4 hours after inoculation, a similar change in resistance in 

 Ul does not occur before 7-8 hours after infection. It has not been possible 

 to decide on the time when the survival curve for Ul changes from an 

 exponential to a multitarget character, except that it is later than for 

 U2. 



The results of these experiments have been tentatively interpreted in the 

 following manner. After making contact with host protoplasm at a suscept- 

 ible site, TMV passes through three different phases before multiplication 

 of the virus occurs. During the first phase, which may last 2 hours at 20°C., 

 or a shorter time at a higher temperature, the infectious entity shows no 

 change in character from its extracellular behavior. The first phase of no 

 change is then followed by a period of about 2 hours, in which the resistance 

 to inactivation by radiation is markedly increased. The explanation we 

 favor is that the virus changes its physical state. 



Phase 2 is followed by another time period of about 1 hour m which no 

 further rise in the resistance of the virus occurs. The meaning of phase 3 is 

 not apparent, although it could be speculated that this is a period required 

 for the infectious nucleic acid to find the proper structure within the host 

 protoplasm required before self-replication can occur. 



Phase 4 represents the start of the virus multiplicative process. WTiile it 

 would not be possible to claim precision, extrapolation of the final slopes of 

 the inactivation curves to the ordinate during phase 4 suggests that the 

 amount of infectious material doubles in amount in about two hours. 



Of particular interest is the failure of the final inactivation slopes of phases 

 3 and 4 to return to the slope encountered in phase 1. The results suggest 

 that the material that is multiplying is different in state than the virus 

 particle which commenced the infection. 



I. Inactivation of Tobacco Mosaic Virus Infectious Nucleic Acid 



The far-reaching discovery by Gierer and Schramm (1956) and Fraenkel- 

 Conrat (1956), that the infectivity of TMV is a property of the nucleic acid 

 portion of the virus particle has provided an additional means for experi- 

 menting with TMV during its earhest stages of reproduction in a local lesion 

 host. First of aU, it could be shown (Siegel et al., 1956) that the differences 

 in UV sensitivity between strains Ul and U2 in the extracellular state dis- 

 appear completely when nucleic acid preparations are tested for viability 

 after being irradiated, as shown by Fig. 4. Secondly, when the two kinds of 

 nucleic acid are rubbed on a leaf, prior to irradiation of the host-virus system, 



