446 H. FRAENKEL-CONRAT 



to circumvent this difficulty (Eraser et al. 1957; Spizizen, 1957), Another 

 instance in which a nucleic acid fraction is held responsible for a viroid effect 

 (mouse leukemia) has been announced (Hays et al, 1957), 



C. Natural Occurrence of Infectious Nucleic Acid 



When infectious nucleic acid is isolated from tissue extracts or culture 

 fluids, even by methods which disrupt many viruses, the possibility must be 

 envisaged that some or all of this nucleic acid could have been present in the 

 free state in that tissue. This would be a logical corrolary to the known occur- 

 rence of viral protein components in plant tissues. Evidence that this can 

 actually be the case was recently obtained by Cochran and Chichester (1957), 

 when they fractionated crude homogenates of TMV-infected Turkish tobacco 

 plants on ion exchange columns. Eor they obtained two widely separated 

 infectious fractions, one of which resembled RNA in its behavior and com- 

 prised a variable proportion of the main infectious fraction (TMV), depending 

 on the age of the infection. 



VI. Chemical and Physicochemical Properties of Infectious 



TMV-RNA 



A. Molecular Weight 



A detailed discussion of the chemical composition of viral nucleic acids 

 win be the subject of a later chapter. Here, only a few general properties of 

 the macromolecidar infectious material will be discussed. Such studies would 

 appear to have little significance, since the low infectivity of all preparations 

 suggests that over 99 % of the material is nonuifectious. However, the re- 

 covery upon reconstitution of about 50 % of the original infectivity seems to 

 exclude that possibility, and to justify the assumption that the main com- 

 ponent of isolated TMV-RNA preparations is actually infectious. 



One of the important physicochemical properties of a bioactive material 

 is its molecular weight. Pertinent data have been obtained by Schuster et al. 

 (1956) and by Gierer (1957) by means of sedimentation, diffusion, viscosity, 

 and other determinations on phenol-prepared nucleic acid. These, as well as 

 earlier Hght-scattering data obtained by Hopkins and Sinsheimer (1955) with 

 heat-salt-prepared nucleic acid of miknown infectivity, have given mole- 

 cular weight values between 0.9 and 4 X 10^. The X-ray sensitive volume 

 of TMV-RNA prepared by the SDS method, as determined by Ginoza and 

 Norman (1957), falls within the same range. Since the calculated weight of 

 the total complement of nucleic acid in a virus particle is about 2.4 X 10^, 

 the various authors favor the idea that the infectious nucleic acid represents 



