THE BIOCHEMISTRY OF PLANT VIRUSES 83 



J. Reconstitution and the Infectivity of Virus Nucleic Acid 

 It has been realized for a considerable time that the nucleic acid of viruses 

 was probably the carrier of the "information" necessary for virus activity, 

 and that it was wrapped in a protein shell in order to render it less liable 

 to the action of extraneous influences. The nucleic acid of the virus may be 

 obtained free, or nearly free, of protein by heating the latter in solution, 

 or by acting on the virus with anionic detergents, or, as has been found 

 quite recently, with phenol. 



The nucleic acid so obtained is highly polymerized and seems to contain 

 all the nucleotides present in one virus particle per nucleic acid particle. 

 That is to say, the particles have a molecular weight of the order of 3 million. 

 This has been determined by light scattering, while the other physical 

 properties are in keeping with this value. If kept, the nucleic acid depoly- 

 merizes spontaneously, presumably because it is sufficiently contaminated 

 with the very stable enzymes which are more or less ubiquitous, and which 

 are invariably present in nucleic acid preparations. 



Some years ago, Fraenkel-Conrat and Williams (1955) were able to demon- 

 strate that the A-protein of Schramm, if allowed to polymerize in the 

 presence of freshly prepared virus nucleic acid, was able to form aggregates — 

 particles resembhng virus in physical properties, and having reasonable 

 stability, w^hich were much more infectious than was either of the con- 

 stituents separately. The order of infectivity was small, even compared with 

 that of the original virus preparations, which are themselves very uninfectious 

 on a weight basis. This caused a certain amount of unfavorable comment 

 and skepticism about the real meaning of the results, but by and large the 

 phenomenon has become accepted because it is repeatable. 



1. Infective Nucleic Acid 



The nucleic acid of the tobacco mosaic virus was itself shown to be capable 

 of initiating infection by Gierer and Schramm (1956a,b) and a similar, but 

 less-detailed study was made simultaneously by Fraenkel-Conrat (1956). 

 The idea that the nucleic acid might be the infectious agent was not a new 

 one, but the general opinion was that it might be too unstable to exist in an 

 infectious form for any time. The fact that the nucleic acid is unstable is 

 easily seen from the failure of Fraenkel-Conrat and Williams (1955) to observe 

 its infectivity in their reconstitution experiments, which were in fact con- 

 trolled by testing the infectivity of the nucleic acid. However, Gierer and 

 Schramm, using a new method for the isolation of the nucleic acid coupled 

 with careful control of temperature, managed to make an infectious nucleic 

 acid preparation. 



The nucleic acid was isolated by shakmg a 10 % solution of the virus in 

 water at pH 7.3 with an equal volume of water-saturated phenol. Phenol 



