10 S. G. WILDMAN 



by the rubbing process. Those that did find the proper site were immediately 

 "fixed" in such a way that they could not be removed by the washing process. 



In this comiection, Siegel (unpubhshed) has found that when uifectious 

 TMV nucleic acid is rubbed on N. glutinosa leaves and the leaf immediately 

 dipped into a strong solution of RNAase, no reduction in lesions occurs. The 

 enzyme was so concentrated as to cause almost instantaneous inactivation 

 of the nucleic acid. This result serves as a further indication of the rapidity 

 with which virus combines with the leaf and becomes protected from the 

 vicissitudes of the extracellular environment. Further discussion of TIVIV 

 infectious nucleic acid will be found in a later section of this chapter. 



Inactivated virus does not compete with infective virus at the site of virus 

 attachment (Rappaport and Siegel, 1955; Rappaport, 1957). Virus whose 

 infectivity had been destroyed by ultraviolet irradiation did not reduce 

 lesion counts even when as much as 10 parts of inactivated virus were present 

 to 1 part of active virus at the time of rubbing. Since inactivation produces 

 no observable change in the structure or composition of the virus, failure of 

 inactivated virus to interfere with the attachment of active virus suggests 

 that attachment is a subtle process that we are far from understanding. 

 Some progress in this direction has come from a kinetic analysis of sero- 

 logical experiments that has led Rappaport (1957) to postulate that anti- 

 bodies covering any one of about 10 critical positions on a virus particle 

 render the particle noninfectious. Such coverage would constitute less than 

 1/100 of the total surface of a virus particle, a fact which further emphasizes 

 the delicate nature of combination of virus and the susceptible site. Later in 

 this chapter, evidence for an exclusion phenomenon will be presented which 

 adds another complication to the understanding of the first process that sets 

 virus activity in motion. 



F. Intracellular Development of Viruses as Deduced from Irradiation 



Experiments 



As the result of applying techniques developed by Luria and Latarjet 

 (1947; Latarjet, 1953) for elucidating the sequence of steps in the process of 

 infection by bacteriophage, it has been possible to gain some knowledge of 

 the intracellular behavior of plant viruses immediately following the start of 

 infection. The experiments are based on the notion that analysis of radiation 

 inactivation curves can reveal changes in the state of the virus. The rationale 

 behind the interpretation of radiation inactivation experiments is found in 

 Stahl's treatment of the subject in Chapter 10 of this volume. 



G. Inactivation of Tobacco Mosaic Virus by Ultraviolet Light 



It has been shown (Siegel and Wildman, 1954) that different strams of TMV 

 may vary about 6 times in their susceptibility to inactivation by ultraviolet 



