560 S. E. LURIA 



As far as the mitiating material in infection is concerned, there is clear 

 evidence that this is the RNA portion, which in the mature virus particles 

 is contained within a spirally assembled shell of protein units (R. E. Frank- 

 lin et al., 1957). The purified RNA fraction extracted from the virus particles 

 can initiate infection by itself, although less efficiently (per unit weight of 

 RNA) than the complete nucleoprotein particles (Gierer and Scliramm, 1956; 

 Fraenkel-Conrat, 1956). 



The early development of the infection, as revealed by changes in the 

 radiation sensitivity of the virus-producing capacity of the infected cells, 

 shows significant differences between infection with complete virus and 

 infection with the RNA fraction alone (Siegel et al., 1957). In infection with 

 complete virus, there is an early phase during which the sensitivity to ultra- 

 violet light is very high. In infection with the viral RNA alone, this early 

 phase is missing; the whole situation evolves as though the process started 

 directly at a later stage. These observations suggest that in infection with 

 complete virus particles a first necessary step is the release of RNA from 

 its protein shell, so that it can act as the 'primum. movens in the process of 

 virus multiphcation. 



It seems probable that with TMV infection, as with phage infection, the 

 multiplying form of the virus consists of nucleic acid not associated with the 

 protein found in the mature product. Here again, the viral protein may be a 

 specific product of the virus-infected cell, utilized for coatmg the essential 

 nucleic acid and providing it with a protective apparatus that enhances its 

 chances of successful transmission to other plants. It is conceivable that 

 transmission of virus from cell to cell within an infected plant may occur by 

 the transfer of RNA elements, rather than of complete nucleoprotein 

 particles. 



B. TMV Protein and Virus Maturation 



What is known of the properties and biosynthesis of TMV protein fits the 

 hypothesis that we have outlined. In infected cells, TMV protein is found, 

 not only in the virus particles, but also as a noninfectious material, pre- 

 sumably not associated with viral RNA or at least readily separated from it 

 by extraction (Jeener, 1956). Isotopic experiments show that at least some 

 of this noninfectious viral protein behaves as a true precursor of the virus 

 particles, into which it lecomes incorporated (Van Rysselberge and Jeener, 

 1957). 



The viral protein is made up of small subunits, about 17,000 in molecular 

 weight. These appear to be uniform in structure and composition, at least 

 within the limits of present analytical methods (Knight, 1957). The protein 

 extracted from infected ceUs or from virus particles has a remarkable ten- 

 dency to aggregate, under suitable conditions, either alone or around a core 



