INFECTIVITY OF TOBACCO MOSAIC VIRUS 445 



to about pH 8 of the pH of optimum stability also differentiates the isolated 

 nucleic acid from intact TMV. 



Finally, an important functional difference was revealed by the studies of 

 Siegel et al. (1957). These authors found that the development of lesions after 

 inoculation with TMV could be largely forestalled by ultraviolet irradiation 

 of the inoculated leaves within the first 2-5 hours, this time span varying for 

 various strains. In contrast, no such protracted sensitive period existed for 

 nucleic acid-infected leaves. This time interval was therefore tentatively 

 interpreted as the time required by the plant for the separation of the nucleic 

 acid from its protein shell. Similar conclusions were drawn from recent ex- 

 periments in which the rate of lesion appearance was compared after inocula- 

 tion with TMV or its nucleic acid, respectively (Fraenkel-Conrat et al., 1958). 

 The concept that the free nucleic acid would rapidly initiate the replication 

 process in a manner not allowing for its continued presence is in accord with 

 other data on the mode of infection by bacteriophage and various viruses. 



B. Other Viruses 



The splitting and reconstitution of TMV and the recognition and accept- 

 ance of its RNA as the infectious moiety have led to an intensive search for 

 similar evidence with other viruses. At present, only a few animal and bac- 

 terial viruses have yielded some success; since these wiU be discussed in later 

 chapters, the pertinent experiments will be only briefly listed here. Colter 

 et al. (B. B. Colter et al., 1957; J. S. Colter et al., 1957) have appHed the 

 phenol method to several virus-infected tissues and have isolated RNA 

 fractions containing viral infectivity. Several of the criteria Msted above were 

 apphed to differentiate the isolated infectivity from that of the corresponding 

 intact virus. In some instances, sedimentability (i.e. precipitability) in MNaCl 

 was an additional criterion differentiating the RNA from the virus. Wecker 

 and Schafer (1957) have performed similar experiments with Eastern equine 

 encephalitis virus-infected tissue. Naturally, studies in which neither the 

 starting material nor the final nucleic acid fraction is purely or even largely 

 viral in nature must be regarded as no more than preliminary. But they 

 suggest the possibility that techniques can be developed to separate all kinds 

 of viruses into native protein and infectious nucleic acid, and possibly to 

 reconstitute virus from such components. It remains to be elucidated whether 

 the protein fractions can be used as noninfectious antigens, and whether the 

 nucleic acids can in some manner be attenuated or genetically modified to 

 render them advantageous for use in vaccines. 



Other studies searching for iofective DNA have centered on bacteriophages. 

 In this instance, the protein supplies the seemingly crucial apparatus for 

 infection of bacteria, but the use of protoplasts as hosts may serve as a means 



