34. THE RIBONUCLEIC ACIDS OF VIRUSES 



287 



TIME AFTER INFECTION (HOURS) 

 Fig. 11. The synthesis of tobacco mosaic virus. Increase of virus concentration 

 after infection with tobacco mosaic virus (X — X — X) and virus ribonucleic acid 

 # — # — #) in Nicotiana tabacum. [G. Schramm and R. Engler, Nature 181, 916 

 (1958).] 



strated by this procedure. It is only possible to determine the total amount 

 of infectious material synthesized if ribonuclease degradation can be 

 avoided. This can be accomplished if the homogenization is performed in 

 the presence of phenol which immediately destroys all enzyme activity. 133 

 Engler and Schramm 134 employed this technique to study the multiplica- 

 tion of TMV. The total infectious RNA in infected leaves (Nicotiana 

 tabacum) was determined by homogenizing the material in the presence of 

 phenol. The RNA which was already enclosed in protein to form intact 

 virus was determined by homogenizing the leaves in the absence of phenol, 

 followed by incubating the homogenate at 37°C. for 1 hour to destroy all 

 free RNA. Only the RNA within the virus remains intact after this treat- 

 ment. Then the protein was extracted with phenol and the liberated RNA 

 measured. The infectivity of the total RNA and the virus RNA was as- 

 sayed by local lesion test on Nicotiana glutinosa. 



The results of this experiment are shown in Fig. 12. A large amount of 

 free RNA is synthesized before new intact virus is produced. The incorpora- 

 tion of this free RNA into virus protein to form intact virus starts some 10 

 hours after RNA multiplication has commenced. Synthesis of virus protein 

 starts several hours later than synthesis of virus RNA, but must be much 



133 R. Engler, Doctoral thesis, Tubingen (1957). 



134 R. Engler and G. Schramm, Nature 183, 1277 (1959). 



