54 KNIGHT 



1 . A similarity in host range 



2. Coincidence of general chemical and physical properties 



3. Possession of common size and shape 



4. Positive cross-interference tests 



5. Positive serological cross reactions 



6. Similar response to genetic change in host 



7. Similar method of transmission. 



The more of the above criteria satisfied, the stronger is the argument 

 for strain relationship. On the other hand, failure to satisfy one of the 

 criteria need not preclude strain relationship. 



Some strains of TMV have been shown to differ in protein com- 

 position. 



Relation of Chemical Structure to Biological Activity. — A number 

 of derivatives of TMV have been synthesized in which known chemi- 

 cal reagents have been coupled to active groups of the virus. In some 

 cases it was found that 70% of the amino groups and 20% of the phe- 

 nol plus indole groups could be substituted before measurable loss of 

 biological activity was observed. Further substitution caused loss of 

 activity. Subsequent studies on strains of TMV have suggested that the 

 biological activity probably does not reside in the surface active groups 

 per se but rather involves some more subtle and at present obscure 

 structural characteristic of the virus. 



The importance of nucleic acid in plant virus reproduction has been 

 emphasized by the findings of Markham and associates wdth turnip 

 yellow mosaic virus. Preparations of this virus were obtained contain- 

 ing essentially 2 components, one a macromolecular protein and the 

 second, a macromolecular nucleoprotein. The protein constituent of 

 the nucleoprotein appeared to be identical with the free macromolec- 

 ular protein, but only the nucleoprotein was infective {Parasitology, 

 1949,3^,330). 



