362 AKNTJAL REPORT SMITHSONIAN INSTITUTION, 1955 



hence that this unique characteristic is not dependent upon the inter- 

 action of two or more molecules in some new and unknown manner. 

 This does not mean, however, that such interaction may not occur 

 within living cells, but only that at one stage a single molecule can 

 carry the complete message. 



A few other viruses have been purified and subjected to similar cor- 

 relation experiments, although usually to a lesser extent than for to- 

 bacco mosaic virus, and the respective characteristic particles are now 

 generally accepted as the respective viruses. These other viruses have 

 been found to exist in different sizes and shapes which are highly char- 

 acteristic in each case. Some are pure nucleoproteins, and others are 

 more complex entities. Some can be obtained in crystalline form, such 

 as, for example, tomato bushy stunt virus, crystals of which are shown 

 in plate 1, figure 2. Then, too, the advent of the electron microscope 

 around 1940 was of the greatest importance to virus research since this 

 instrument permits pictures to be taken of objects in the size range 

 of the viruses, namely 10 to about 300 mjw, a range not covered by the 

 optical microscope and the range of the unknown void between the 

 organisms of the biologist and the molecules of the chemist. Although 

 information regarding approximate sizes and shapes of viruses had 

 been obtained earlier by indirect methods, for the first time it became 

 possible really to see the viruses. For example, the individual par- 

 ticles or molecules that go to make up the crystals of bushy stunt 

 virus were found by means of the electron microscope to be spheres 

 30 mjtt in diameter. The individual molecules of tobacco mosaic virus, 

 seen in plate 1, figure 3, were proved to be 15 m/x. by 300 m/i, values in 

 good agreement with those reached earlier by means of indirect 

 methods. 



From the outset the chemical studies on tobacco mosaic virus have 

 been designed primarily to elucidate the nature of the chemical struc- 

 ture that is responsible for virus activity. The virus is stable between 

 about ^H 2 and pH 8. In dilute alkali or on treatment with deter- 

 gents the protein and nucleic acid are split apart and the activity is 

 lost irreversibly. In general, it has been found that the least destruc- 

 tive treatments which have resulted in separation of the nucleic acid 

 have been those in which hydrogen-bond -breaking conditions have been 

 coupled with salt treatment. Each of the components of this unusual 

 nucleoprotein have been subjected to extensive analysis. The protein 

 component comprising 94 percent of the structure was found to be 

 composed of 16 of the amino acid building blocks common to most 

 proteins. Regardless of the source of the virus, the relative amounts 

 of these amino acids were constant. No o-isomers or unusual amino 

 acids were found. Unlike sperm nucleoproteins, tobacco mosaic virus 

 was found not to contain an excess of basic amino acids. As a matter 



