EEPRODUCTION OF VIRUSES: A COMPARATIVE SURVEY 561 



of nucleic acid, to give the typical helical arrangement of tlie protein in the 

 virus particle (Schramm, 1947). 



Complete virus particles can be reconstituted by recombining RNA and 

 protein separately extracted from virus. The reconstituted particles have 

 some infectivity (Fraenkel-Conrat and Williams, 1955). If RNA from one 

 virus strain and protein from a diflFerent strain are combined, the progeny to 

 which they give rise has the genetic characteristics contributed by the 

 RNA. 



Thus, the TMV protein appears to be a specific material, without intrinsic 

 genetic function, produced under the genetic control of the viral RNA, 

 and utihzed in the morphogenetic process of virus maturation. 



Here again, this time with a typical RNA virus, the nucleic acid must be 

 considered as the primary genetic material of the virus, and the mature 

 particle as one product of the genetic activity of the virus. The occurrence 

 of other virus-related proteins, which are probably not precursor proteins, 

 indicates that the mature virus is not the only specific product of virus- 

 infected cells. Which other cell functions this virus may control is not 

 known. 



The amomit of RNA in a TMV particle can probably carry more genetic 

 information than is needed to determine the specificity of the viral protein. 

 The additional information, if any, may control other functions of the virus 

 in the cell. We may also find, in such an RNA virus, some "transduced" 

 elements of host cell RNA. 



C. Other RNA Viruses 



A few scattered observations on other RNA-containing viruses support 

 the conclusions reached for TMV virus. With poliovirus, Mengo, and West 

 Nile encephalitis viruses, successful transmission of infection has been 

 reported by means of an RNA fraction extracted from infected cells (Colter 

 et al., 1957a,b). Conversely, with turnip yellow mosaic virus, there is found 

 in iafected cells a fraction of particles, similar to the infectious virus particles 

 in size, structure, and protein composition, but without RNA and com- 

 pletely noninfectious (Markham and Smith, 1949). These particles are prob- 

 ably a product of faulty maturation, the essential RNA failing to be enclosed 

 into the proteiu shell. In the complete virus particles, as well as in the nonin- 

 fectious ones, the protein actually appears to constitute a shell composed of 

 repeated subunits (Klug et al., 1957). It seems a useful hypothesis to assume 

 that with these viruses, and probably also with others like poliovirus, whose 

 particles contain only RNA and protein, the synthesis of virus protein is 

 always a terminal event, leading to the maturation of the virus and to the 

 cessation of the reproduction of its essential genetic material. 



