VIRUSES — STANLEY 271 



of tobacco mosaic vims are shown in plate 6. Some progress has been 

 made in studies on methods for changing the chemical structure of 

 tobacco mosaic virus without causing a loss of virus activity. Dr. 

 Anson and I found that the sulfhydryl groups of the virus can be 

 abolished by reaction with iodine and the altered virus still retains its 

 normal biological activity as shown by the number of lesions it causes 

 on Nicotmna glutinosa plants and by the characteristic disease pro- 

 duced in Turkish tobacco plants. Since the virus isolated from the 

 latter plants had the normal number of sulfhydryl groups, the struc- 

 tural change caused by iodine treatment was not perpetuated in sub- 

 sequent generations of the virus. Because of the possibility that the 

 iodine-altered virus might be reduced to normal virus within the 

 plant cells, other reactions of a less readily reversible nature were 

 sought. With Dr. Miller it was demonstrated that most of the amino 

 groups of tobacco mosaic virus may be acetylated by means of ketene 

 without causing a measurable change in the specific virus activity or 

 in the nature of the disease produced in Turkish tobacco plants. Since 

 it seems unlikely that the acetyl groups are removed on inoculation of 

 the modified virus to plants, the fact that the virus produced in such 

 plants contains the normal amount of amino nitrogen may be regarded 

 as evidence that the modified virus actually brings about the production 

 of normal or unmodified virus. Similar results have been obtained 

 with virus modified by the introduction of about 3,0G0 phenylureido 

 groups per molecule of virus by means of reaction with phenyliso- 

 cyanate. These results demonstrate that a large portion of the surface 

 structure of the virus may be changed without interfering with the 

 basic reaction of virus reproduction. Other reactions are being 

 studied in an effort to secure modifications that will be perpetuated 

 in subsequent generations of the virus. The purposeful production 

 of new and useful strains by chemical means is one of the major prob- 

 lems in the virus field and its solution will be of tremendous im- 

 portance not only from a practical standpoint but also in connection 

 with the larger and fundamental problem of the nature of virus ac- 

 tivity. The latter problem, the inactivation problem and the problem 

 of induced mutation are all so closely related tliat it is impossible to 

 attack one without attacking the others and simultaneously funda- 

 mental problems in other fields, such as the origin of a cancerous cell, 

 the duplication of a chromosome, the mutation of a gene, and even 

 perhaps the nature of that ill-defined something called life. 



Although we do not know how viruses originate, reproduce, or 

 mutate, we have learned much about their chemical properties during 

 the past 5 years. We know that for every reasonably stable virus 

 which has been investigated there is a definite, characteristic, high 

 molecular weight material which is at least as complex as a nucleo- 



