34. THE RIBONUCLEIC ACIDS OF VIRUSES 285 



between RNA and protein in strain Ul is revealed, also, by other studies. 

 [See Section III, d, (1).] 



After direct inoculation of RNA, or after the release of RNA from its 

 protective protein shell, we may imagine that the RNA migrated from the 

 surface to the inside of the cell. Here it may be shielded from the damaging 

 effects of UV light. A further possibility is that RNA combines with some 

 host cell protein which may offer better protection against UV irradiation. 

 It has already been mentioned that the type of RNA-protein bonding may 

 be important for the resistance to UV irradiation. 



Before new infectious units appear (phase 4), a plateau of resistance to 

 UV is maintained (phase 3). During phase 4, a second rise in resistance 

 occurs. The timing of all four phases is markedly dependent on tempera- 

 ture. 



(2) Effect of Ribonuclease on Virus Multiplication. As long as the RNA is 

 enclosed in its protein covering, it is inaccessible to the enzymic action of 

 ribonuclease. 126 Upon release from its protective protein, it is accessible to 

 enzymic degradation. The virus RNA must be separated from the virus 

 protein at some time, in order that the RNA may induce its reduplication. 

 Therefore, it should be possible to obtain some information on the course 

 of infection by adding ribonuclease to infected cells in order, artificially, to 

 increase the ribonuclease concentration within the cell. 



Casterman and Jeener 127 • 128 infiltrated leaves of Nicotiana tabacum with 

 ribonuclease in vacuo either before or at various times after inoculation with 

 virus and investigated the resulting action of the enzyme on virus multi- 

 plication. If the infiltration of ribonuclease occurred before infection or up 

 to 15 minutes after infection, no virus multiplication occurred. If ribo- 

 nuclease was infiltrated into the cells 2 hours or more after infection, it was 

 no longer possible to inhibit virus multiplication. Simple immersion of the 

 leaves in the ribonuclease solution, followed by superficial washing with 

 water, had no influence on virus multiplication. 



If one postulates that ribonuclease inhibits virus multiplication only by 

 splitting the free virus RNA, these studies can be interpreted by assuming 

 that virus RNA is freed of its protein directly after infection. Two hours 

 after infection, the RNA must be present in a form unaccessible for ribo- 

 nuclease, possibly by association with some cell structures or by the forma- 

 tion of new intact virus. On the other hand, the inhibitory action of ribo- 

 nuclease can also be due to the formation of a complex with intact virus. 

 Loring 126 and Kleczkowski 129 have reported that TMV can form nonin- 



126 H. S. Loring, J. Gen. Physiol. 25, 497 (1942). 



127 C. Casterman and R. Jeener, Biochim. et Biophys. Acta 16, 433 (1955). 



128 C. Hamers-Casterman and R. Jeener, Virology 3, 197 (1957). 



129 A. Klezkowski, Biochem. J. 40, 677 (1946). 



