THE STRUCTURE OF VIRUSES 



123 



index of the amount of virus applied, a 

 very accurate method for estimating the 

 concentration of this virus was developed. 

 By means of this method differences in 

 virus concentration of 10 per cent or 

 greater can be detected without undue 

 effort. The fact that tobacco-mosaic virus 

 can be titrated accurately made possible 

 the correlation of virus activity with the 

 chemical and physical properties of the 

 protein. This was of paramount impor- 

 tance, because the demonstration, beyond 

 a reasonable doubt, that the virus prepara- 

 tions (or at least the major component in 

 each of the preparations) actually consisted 

 of virus, and the further consideration and 

 the acceptance of the purified preparations 

 as virus were directly dependent upon such 

 work. Although much of the work done 

 since the isolation of the purified virus 

 preparations has been devoted to a study 

 of this problem, there will be presented 

 here briefly only the more significant re- 

 sults, most of which have been obtained 

 with tobacco-mosaic virus. This virus has 

 one of the widest host ranges known, for 46 

 different species of plants, representing 14 

 widely separated families, are susceptible 

 to the mosaic disease. Although attempts 

 to obtain purified preparations from all of 

 these have not been made as yet, it is of 

 considerable significance that virus prepa- 

 rations possessing essentially the same 

 chemical, physical, and biological proper- 

 ties have been obtained from different 

 batches of diseased Turkish tobacco. Bur- 

 ley tobacco, tomato, phlox, spinach, petunia, 

 and nightshade plants. This is a definite 

 indication that infection of widely differ- 

 ent hosts with the same virus is followed 

 by the production in these different hosts 

 of the same nucleoprotein, a material which 

 is characteristic not of the host but of the 

 disease. The yield of nucleoprotein ob- 

 tained was found to vary widely depending 

 upon the host, for Turkish tobacco plants 

 gave a yield of 2 to 3 mg of purified virus 

 per cc of juice, tomato plants about 1 mg 

 per cc, and spinach and phlox plants con- 

 siderably less than 1 mg per cc. It should 

 be noted that some of these plants, such as 



phlox and spinach, are far removed from 

 the tobacco family. For example, no sero- 

 logical relationship was found by Chester 

 between the protein from normal tobacco 

 plants and that from normal phlox plants. 

 The fact that on infection these two differ- 

 ent plants foster the production of the 

 same virus is an indication that virus is 

 not produced by the simple polymerization 

 of serologically active normal proteins. 



There is good evidence that viruses oc- 

 casionally change or mutate during produc- 

 tion in a host and give rise to new strains 

 which may be isolated, grown, and studied 

 apart from the parent virus. Many strains 

 of tobacco-mosaic virus are recognized and 

 to date 4 of these have been isolated in 

 purified form. Preparations of strains of 

 the same virus have been found to have 

 somewhat similar general properties, yet it 

 was found that it is possible to distinguish 

 each of the 4 preparations by means of 

 definite and characteristic chemical, physi- 

 cal, and serological properties. This find- 

 ing is of considerable importance, for it 

 indicates that when a virus changes or 

 mutates, the change is accompanied by the 

 formation of a new and slightly different 

 nucleoprotein. Furthermore, the concen- 

 tration reached in a given host was found 

 to vary widely with the strain of virus. 

 The amounts of the different purified 

 viruses obtainable from a given quantity of 

 starting material have also been found to 

 vary widely. For example, some batches 

 of badly diseased Turkish tobacco plants 

 have been found to contain one part of 

 virus per 200 parts of fresh green plant 

 material, whereas Turkish tobacco plants 

 diseased with cucumber-mosaic-1 virus con- 

 tain only about one part per million as 

 virus. Beard and Wyckoff obtained about 

 one part of papilloma virus per five 

 thousand parts of the starting material 

 (based on the whole rabbit), and Northrop 

 estimated that the crude culture of the 

 staphylococcus with which he worked con- 

 tained about one part per million as bac- 

 teriophage. It should be recognized, there- 

 fore, that the amount of virus occurring in 

 a host may vary tremendously, depending 



