VIRUSES AS MOLECULES 



FIGURE 3 - HEXAGONAL PLATE CRYSTALS OF TOBACCO 

 MOSAIC VIRUS IN BPIDERMAL CELL OF PETUNIA. (H.P. 

 Beale, Contrib. JJoyce Thompson Inst. 8, 413 11937J )• 



It iB interesting to observe that in the interior of the plauit cell tobacco 

 mosaic Tirus crystallize in the form of hexagonal plates but in the beaker in 

 the form of long slender needles. 



The first question to come to mind is whether this macromolecular nucleo- 

 protein obtained from mosaic diseased tobacco plants actually Is the virus or 

 merely some substance contaminated by the virus. When a bacteriologist is 

 confronted with the task of making a decision of this sort, he attempts to 

 apply the criteria suggested by Koch. If the suspected orgsmism is to be re- 

 garded as the etiologlc agent of a given disease, it must always be associated 

 with the disease; it must be isolated in pure culture on artificial media; the 

 disease must be reproduced when a suitable host is inoculated with the pure 

 culture; and finally the organism must be reisolated in pure form from the ex- 

 perimental host. The evidence amstssed by Stanley to prove that the virus pro- 

 tein is the etiologlc agent of tobacco mosaic virus can be organised to follow 

 this generally accepted procedure fairly closely. The viinis protein meets the 

 first of Koch's criteria, for it has been isolated from many different batches 

 of diseased tobacco plants and from other plant species infected with the virus. 

 We know that it is the same protein in all of these cases, because the various 

 preparations possessed roughly the same chemical, physical, biological and im- 

 munological properties. It can be accepted as a fact, then, that the virus 

 protein is always associated with the tobacco mosaic disease. The second 

 criterion can not be applied literally, for the viruses can not be cultivated 

 on artificial media. However, the significance of the second criterion as ap- 

 plied to bacteria is derived from the .fact that this procedure constitutes a 

 demonstration that the organism under question is not contaminated by some other 

 form, that is, that it is pure, it has been possible to apply other tests of 

 purity and to demonstrate beyond reasonable doubt that tobacco mosaic virus 

 protein can be obtained as an essentially pure chemical material. Some of the 

 evidence in support of this conclusion may be summarlaed. The ohemical and bio- 

 logical properties of the material remained unchanged following attempts to 

 fractionate by various procedures; it is not possible to demonstrate the presence 

 of an impurity by sensitive immunological reactions; and the material is essen- 

 tially homogeneous with respect to various physical criteria such as sedimenta- 

 tion rate and electrophoretic behavior. Virus activity has never been obtained 

 free from the protein. The ultraviolet absorption spectrum of the protein co- 

 incides with the destruction spectrum of virus activity, and the pH stability 

 ranges of the protein and of virus activity coincide. This evidence of the 

 purity of tobacco mosaic virus protein seems every bit as conclusive as that 

 usually obtained by bacteriologists in applying Koch's second criterion. The 

 last two criteria can be applied easily, for the virus protein is capable of 

 causing the typical disease symptoms in plamts, of being multiplied within them, 

 and of reisolation from infected plants. Therefore, it seems reasonable to con- 

 clude, on the basis of this evidence, that tobacco mosaic virus nucleoprotein 



