82 R. MARKHAM 



invariably contained nucleic acid, probably of plant origin. Later workers have 

 tended to eliminate this nucleic acid by their manipulations, and there is 

 little doubt that the X-proteiri is normally devoid of this substance. Some 

 workers have identified a number of proteins of this type by working at 

 pH's at which the X-protein is in a state of flux (Coramoner et al., 1953), but 

 there seems to be every reason to think that only one basic substance is 

 involved, which is capable of aggregation and dissociation. 



The precise nature of the X-protein is as yet uncertain (Delwiche et al., 

 1955). It is quite evident though that it resembles the "top" component of 

 the turnip yellow mosaic virus (q.v.) in many ways (van Kysselberge and 

 Jeener, 1957). 



Amino acid analyses which have been made on the X-protein by Newmark 

 and Fraser (1956) indicate that this protein has the same amino acid com- 

 position as the protein isolated from the whole virus. 



I. Effect of Heat on the Virus 



The tobacco mosaic virus has long been recognized as being unusually 

 heat-stable. This reputation has been vastly over-emphasized, largely 

 because the heat denaturation was originally followed by biological methods, 

 and not only is the denaturation an exponential, or nearly exponential, 

 process, but it is now known that the nucleic acid of the virus is itself 

 infectious, and is moreover much less susceptible to the action of heat. 



Actually the virus is denatured relatively readily by temperatures of 

 75°C. or thereabouts, although this is much less than the 36°C. required 

 to denature the A- and X-proteins. In the presence of salt, the nucleic acid 

 is then released from the protein and remains in solution, while the denatured 

 protein clumps and settles from solution (Cohen and Stanley, 1942). The 

 course of this denaturation has been followed by electron microscopy by 

 Hart (1956). The denaturation process resembles the melting of the protein 

 part of the rods from one end (this is additional evidence that the rods are 

 not symmetrical in their length), and eventuaDy all the protein is converted 

 into spherical objects having about the same volume (53,000 /it,^) as the virus. 

 The nucleic acid is ejected during this process, and then, presumably, the 

 spheres of protein clump to give the coagulimi of denatured protein. 



Apart from the action of alkali and dodecyl sulfate, the virus is extremely 

 stable to aU manner of aqueous solutions including many proteolytic enzymes, 

 although, of course, the action of carboxypeptidase is an exception. Solutions 

 of the A-protein and of the denatured virus protein are, however, reasonably 

 readily digested by enzymes, and it is in this way that much of the structure 

 of the virus protein has been elucidated. 



