VIRUS PROTEINS — STANLEY 503 



possesses virus activity, and the essence of virus activity is reproduc- 

 tion. It is this property of the virus proteins that may give us a lead 

 in our consideration of the growth of protoplasm and in which we are 

 especially interested this afternoon. Let us examine the facts that 

 we have at our disposal. The introduction of a few molecules or 

 perhaps even one molecule of tobacco mosaic virus protein into the 

 cell of a susceptible host is followed by the production within a very 

 short time of millions of molecules of the kind introduced. The 

 production is Dot confined to the one cell but spreads to adjoining 

 cells, so that not one cell but millions of cells are involved in the 

 production of the new molecules of virus protein. We know that the 

 new virus protein is of the same kind as that originally introduced, 

 because we can isolate it and compare its properties with the properties 

 of the virus protein introduced. We say that under such conditions 

 the virus protein grows or reproduces, for from a few molecules we 

 secure millions of molecules. Now, what happens when we retain the 

 virus protein in a test tube or attempt to culture it on cell-free media or 

 introduce it into dead cells or the cells of nonsusceptible hosts? We 

 know that nothing happens, the amount of the protein does not in- 

 crease, reproduction does not occur. Viruses have been found to 

 reproduce, therefore, only within the living cells of certain susceptible 

 hosts. I have said that in such cells the introduction of one kind of 

 molecules results in the production of millions of the same kind. This 

 is true, but it is not the whole truth, for just as there is this remarkable 

 tendency for the production of millions of molecules of the same kind, 

 there is also a remarkable tendency for the production of a very few 

 slightly different molecules. The subsequent reproduction of such 

 slightly different molecules gives rise to what we call a new strain, and 

 the whole phenomenon may be regarded as mutation. We recognize 

 that such a change has occurred by the symptoms we observe and 

 because it is possible by suitable technique to isolate such strains. 

 Thus Jensen has been able to isolate from a pure strain of tobacco 

 mosaic virus over 50 strains, each of which he considers to be different 

 and distinctive, and Holmes has been able to isolate a strain of tobacco 

 mosaic virus that is so mild in its effect on the Turkish tobacco plant 

 that it is difficult to determine its presence by mere observation. It 

 might be argued that these various strains existed in the so-called pure 

 strain as contaminants, but all the evidence obtained by Jensen, 

 Kunkel, Holmes, and McKinney indicates that the new strains arise 

 from a single pure strain when reproducing within the plant cells. We 

 may conclude, therefore, that tobacco mosaic virus protein may 

 reproduce within the cells of certain hosts and that when it does it 

 tends to mutate and to give rise to new strains. Now, this ability 

 to reproduce and to mutate only when within the living cells of certain 



