CHEMICAL STUDIES ON VIRUSES — STANLEY 365 



mosaic virus never kills Turkish tobacco plants, yet the J14D1 strain of 

 this virus kills every young Turkish tobacco plant which it infects. 

 Viruses can mutate, and the accompanying change in disease mani- 

 festation can be exceedingly drastic. Therefore, if a host contains a 

 virus, even though a very mild or latent virus, the possibility of the 

 formation of a very virulent or even lethal virus strain always exists. 



Attempts to achieve mutation by changing the structure of tobacco 

 mosaic virus by means of known chemical reactions have been made 

 from time to time during the past 15 years. Although acetyl, carbo- 

 benzoxy, phenyl ureido, and chlorobenzoyl derivatives of tobacco mo- 

 saic virus have been prepared and found to be biologically active, no 

 heritable mutation was achieved, for the progeny resulting from infec- 

 tion of plants with such derivatives always consisted of ordinary to- 

 bacco mosaic virus. It was also found that the sulfhydryl groups of 

 tobacco mosaic virus could be abolished by treatment with iodine with- 

 out affecting the ability to produce ordinary tobacco mosaic virus. 

 Furthermore, experiments by Knight and by Fraenkel-Conrat in the 

 Virus Laboratory have indicated that the removal of about 2,600 thre- 

 onine residues or the addition of about 1,000 leucine residues per mole 

 of tobacco mosaic virus has no apparent effect on the infectivity, the 

 sj^mptoms, or the nature of the progeny on infection of susceptible 

 hosts with such derivatives. Thus, although these results indicate that 

 various groups including amino acids can be added to or removed from 

 the virus structure without measurably altering the biological activity, 

 the fact that, in nature, structural changes do accompany the formation 

 of mutant strains leads one to expect that sooner or later a structural 

 change by means of a known chemical reaction will prove to be herit- 

 able and hence will represent a mutation. Needless to say, this result, 

 if achieved, will be of the greatest importance. 



Now, in order to place tobacco mosaic virus in proper perspective 

 with viruses generally, I should like to close by presenting a bird's-eye 

 or electron-microscope view of a series of viruses having different sizes 

 and shapes. The structures shown in plates 2 and 3 are all at the same 

 magnification and range from the large elementary bodies of vaccinia 

 which are around 300 nifi in size down to the small particles of bushy 

 stunt virus which are around 30 mfi. Thus, this series overlaps ac- 

 cepted living organisms at one end and approaches very closely to 

 accepted protein molecules at the other end and actually serves to fill in 

 the void that formerly existed between the organisms of the biologist 

 and the molecules of the chemist. 



In figure 1 of plate 2 are shown the elementary bodies of vaccinia 

 which occur in the vaccine used to protect against smallpox. This 

 virus is considered to be a strain of smallpox virus, and here we have 

 an example of a mutant or altered strain of a virus which has been 



