THE STRUCTURE OP VIRUSES 



131 



from viscosity studies aud from the molecu- 

 lar weight of about 50 xlO°, is 12.3 mp, 

 which is in agreement with the results of 

 ultrafiltration, if it be assumed that the 

 width of the particle represents the limit- 

 ing dimension. This value for the cross 

 section is approximately the same as that 

 of 15.2 mn suggested by Bernal on the basis 

 of X-ray data and that of 12.5 m|j sug- 

 gested by Langmuir and Schaefer as a 

 result of studies on monolayers of the vi- 

 rus. Lauffer's general conclusions regard- 

 ing size and shape of the particles of to- 

 bacco-mosaic virus were confirmed recently, 

 when by direct observation by means of the 

 electron microscope, Kausche, Pfankuch 

 and Ruska found that most of the particles 

 in a dilute solution of a chemically pre- 

 pared sample were about 330 mp in length 

 and about 12 to 15 mp in cross section. 



Bawden and Pirie found that the purifi- 

 cation of tobacco-mosaic virus by chemical 

 means resulted in a loss in activity and 

 filterability and an increase in stream 

 double refraction, and concluded that the 

 purification had caused an aggregation of 

 the virus. Similar results have been re- 

 ported in the cases of other viruses, and as 

 a result there has been a tendency on the 

 part of virus workers to assume that purifi- 

 cation as such must invariably cause aggre- 

 gation of virus and hence that it is im- 

 possible to isolate virus in an unaltered 

 condition. The question of the aggregation 

 of tobacco-mosaic virus was studied by the 

 writer with Loring and Lauffer, and it 

 was found that virus purified by rapid 

 chemical treatment in the cold, or prefer- 

 ably by means of ultracentrifugation in 

 the cold, was entirely comparable to the 

 virus in untreated juice with respect to 

 specific activity, filterability, and stream 

 double refraction. However, merely allow- 

 ing such purified virus to stand for a short 

 time at room temperature and in the pres- 

 ence of low concentrations of salt was 

 sufficient to cause aggregation. Similar re- 

 sults have been obtained with latent-mosaic 

 and tobacco-ring-spot viruses. It should 

 be recognized, therefore, that although vi- 

 ruses may be aggregated readily by chemi- 



cal treatment and most of the chemically 

 purified virus preparations that have been 

 isolated have consisted of aggregated vi- 

 rus, it is possible nevertheless to isolate 

 tobacco-mosaic virus which is comparable 

 to virus in unpurified preparations and 

 which resembles it in specific activity, 

 filterability, and stream double refraction. 



The occasional suggestions that in un- 

 treated juice tobacco-mosaic virus consists 

 of small units, or that active low-molecular- 

 weight material may be produced following 

 a given treatment, are not in accord with 

 results obtained in the writer's laboratory. 

 If active low-molecular-weight material ex- 

 isted, it would not be possible to sediment 

 the active units in a centrifuge at a speed 

 at which other low-molecular-weight ma- 

 terial, such as hemoglobin, fails to sedi- 

 ment. However, it is an experimental fact 

 that when untreated juice or preparations 

 which are supposed to contain low-molecu- 

 lar-weight active material are mixed with 

 hemoglobin and centrifuged at a speed 

 which will sediment ordinary tobacco- 

 mosaic virus but not hemoglobin, the virus 

 activity is found in the sediment and not 

 with the unsedimented hemoglobin in the 

 supernatant liquid. This is good evidence 

 that virus in untreated juice does not 

 consist of low-molecular-weight material. 

 Similar results have been obtained by 

 Hughes, Pickels and Horsf all in a study on 

 the differential centrifugation of proteins. 

 These workers demonstrated that hemo- 

 cyanin can be separated from egg albumin 

 and that yellow-fever virus can be sepa- 

 rated from serum protein without any 

 difficulty by means of differential centrifu- 

 gation. It may be concluded that to date 

 there has been no convincing demonstration 

 of the existence or production of low- 

 molecular-weight material carrying virus 

 activity. 



The isolation of several of the viruses in 

 the form of high-molecular- weight proteins, 

 and the demonstration beyond a reasonable 

 doubt that the preparations are essentially 

 pure and that virus activity is a specific 

 property of the proteins, necessitate a con- 

 sideration of virus activity in terms of these 



