508 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



that are catalyzed by intracellular enzymes. In this connection it is 

 interesting to consider that the final step to which I just referred may 

 consist of chemical reactions catalyzed by the virus protein itself. It 

 may be regarded as being similar to an ordinary enzymatic reaction 

 except that in this case the intracellular reaction is catalyzed by the 

 same kind of molecules that are introduced. It is obvious that for 

 the successful operation of this mechanism there must be present in 

 the protoplasm all the necessary component parts. If one or more is 

 missing, the virus protein cannot be built up. This serves to explain 

 the specificity of viruses, for a virus would be expected to reproduce 

 only in protoplasm containing the component parts of the virus mole- 

 cule introduced. There is some evidence that tobacco mosaic virus 

 protein is built from such component parts and not from the large 

 proteins normally built up in such cells, for this virus protein repro- 

 duces itself in both Turkish tobacco and phlox plants, and it has been 

 found by sensitive serological methods that the proteins normally 

 existing in these plants are quite different. This is an indication 

 that the virus protein is built up from smaller serologically inactive 

 units that must be common to both plants. 



Now, if we may assume this mechanism for the reproduction of 

 the virus proteins, may we not assume a similar mechanism for the 

 production of all the proteins that are synthesized within the cell, and 

 hence for the growth of protoplasm? We know that some virus pro- 

 teins reproduce quite rapidly and others more slowly, hence there 

 must in the first instance be a tendency for the reaction to proceed 

 rapidly and in the latter instance for it to proceed more slowly. 

 Whether this tendency be real or result from the relative lack of a 

 given component, if it exists, it would, together with the mechanism 

 that has been discussed, be sufficient to explain not only the various 

 factors involved in the reproduction of the virus proteins but also the 

 orderly manner of the growth of protoplasm. There could be explained 

 in this manner the reproduction of virus proteins, their reproduction 

 only in certain living cells, the rapid rate of reproduction in some cases 

 and the slow rate in others, the change or mutation of one strain into 

 one or more other strains, the immunity from other strains that results 

 from infection with one strain and the failure of one virus to immunize 

 against a different one. Furthermore, we see that the possibility of 

 synthesizing virus proteins in the absence of living cells is not denied us, 

 for such synthesis should occur provided the necessary components 

 are provided and the proper conditions are achieved. If we are ever 

 able to synthesize virus proteins in the absence of living cells, then we 

 shall have gone a long way toward the synthesis of protoplasm. 



I should, perhaps, remind you at this point that I am speaking of 

 fancy and not of fact. I think it well occasionally to assemble all the 

 facts at our disposal, to use them as a foundation and then to allow 



