34. THE RIBONUCLEIC ACIDS OF VIRUSES 295 



protein and an internal nucleoprotein ring having RNA on its external 

 surface. 161 - 162 In the case of Newcastle disease virus (NDV), which is also a 

 myxovirus and which is considerably larger than influenza and fowl plague 

 viruses, the RNA is probably arranged on a ringlike structure that is sur- 

 rounded by a trypsin-sensitive shell. 162 The phospholipid of the virus seems 

 to be superficially situated. 



3. Preparation of Biologically Active Ribonucleic Acids 



An infectious RNA component has now been isolated from 8 different 

 animal viruses. These are : Mengo and West Nile encephalitis viruses, equine 

 encephalomyelitis virus (type east and west), poliomyelitis virus, mouse 

 encephalomyelitis virus, murine encephalomyocarditis virus, foot and 

 mouth disease virus, Semliki forest virus. 



In all cases, the RNA component has been isolated from tissues or cells 

 infected with the virus in question and in the case of poliovirus, mouse 

 encephalomyelitis virus, and western equine encephalomyelitis virus, 

 nucleic acid has also been isolated from highly purified virus preparations. 



a. RNA form Mengo and West Nile Encephalitis Viruses 16 *' 164 



(1) Preparation. Mengo and West Nile viruses were grown in cells of the Ehrlich 

 ascites carcinoma. A saline suspension of samples of mouse brain, which had been 

 infected with the virus in question, was injected intraperitoneally into mice in which 

 the carcinoma had grown for several days. After some days, the infected cells were 

 collected, washed, and frozen. The frozen tissues were ground to a fine powder, mixed 

 with buffer at pH 7, and centrifuged. The nucleic acid was extracted from the super- 

 natant by the phenol method. 



The infectivity of the aqueous phase, which contains the extracted nucleic acid of 

 virus and cells, was determined for both viruses by intracerebral injection into mice. 

 The viruses produced were recovered from the infected mouse brains and identifica- 

 tion was accomplished by means of specific immune sera. 



The infectivity of nucleic acid preparations was of the order of 0.1% of the virus 

 suspensions from which they were prepared. This is similar to the infectivity of 

 TMV-RNA. Virus RNA may occur in infected cells in a free form synthesized by the 

 host cells in response to virus infection, in intact virus particles, or in both forms in 

 the same cell. In order to distinguish between these possibilities, a homogenate of 

 tumor cells, infected with Mengoencephalitis, was incubated with ribonuclease before 

 phenol treatment. This results in a degradation of any virus RNA or precursor of 

 this which is present in free form, as well as cellular RNA. However, this treatment 

 had little if any effect on the infectivity of the active component which was isolated 

 with phenol after the ribonuclease treatment. Thus, the infectious RNA probably 

 originates from intact virus particles. 



(2) Proof thai Infectivity Was Due to RNA. The nitrogen/phosphorus ratio of 



161 R. C. Valentine and A. Isaacs, J. Gen. Microbiol. 16, 195 (1957). 



182 R. C. Valentine and A. Isaacs, J. Gen. Microbiol. 16, 680 (1957). 



1(3 J. S. Colter, H. H. Bird, and R. A. Brown, Nature 179, 859 (1957). 



»«« J. S. Colter, H. H. Bird, A. W. Moyer, and R. A. Brown, Virology 4, 522 (1957). 



