80 R. MARKHAM 



the virus nucleic acid for the basic groups of the virus, and so allows the 

 protein to become free. Certainly one of the most intriguing things which 

 it does do is to start from one end of the virus rods, thus showing dramatically 

 that the rods have two different ends. As the attack proceeds the virus is 

 peeled off and the nucleic acid is allowed to protrude. This can be observed 

 by electron microscopy (Hart, 1955b), and the composition of the thread can 

 be determined by acting on it with enzymes. It is, in fact, unstable to ribo- 

 nuclease. Although a substantial proportion of the protein coat may be 

 removed, the virus retains its infectivity unless the nucleic acid is destroyed 

 by some means or other. 



Schramm et al. (1955a, b) used another of Pirie's techniques to degrade 

 tobacco mosaic virus. Pirie noted that the virus was very unstable to dilute 

 alkali (Bawden and Pirie, 1937a, 1940) and Schramm et al. used carefully 

 adjusted alkaline solutions to degrade the virus. The pH required is about 

 10.3 and the protein is stripped off either at or 35°C. Presumably because 

 the — 0H~ ions are so small, the attack proceeds all over the surface of 

 the virus, and small native protein fragments are released. There is some 

 indication, however, that certain regions on the virus rods are more suscept- 

 ible to the action of alkali, but the dissolution is complicated by the tendency 

 of the small protein particles to reaggregate (Harrington and Schachman, 

 1956). In this case, too, the nucleic acid is revealed as an axial thread, and 

 while the latter is intact, the rods, eaten away as they are, still retain their 

 infectivity. The alkaline solutions also tend to break down the virus nucleic 

 acid, but at pH 10.3 the action is very slow. 



The position of the nucleic acid was determined still more accurately by 

 Frankhn (1956a), who, by X-ray diffraction, measured the radial density 

 distribution of the virus rods and also of the nucleic acid-free polymerized 

 A protein (which will be mentioned later). She found that the density due 

 to the phosphorus atoms of the nucleic acid was located at about 40 A from 

 the axis of the rods. This distance is well within the part of the rod occupied 

 by protein, which extends to within 19 A of the axis, so that the nucleic acid 

 is in fact buried in the protein. This is a very important observation, because 

 it means that the two components must share certain structural regularities. 



The nucleic acid contributes positively to the birefringence of the rods, 

 so that the structure of the virus nucleic acid is very unhke that of deoxy- 

 ribonucleic acid (Franklin, 1955b), which is strongly negatively birefringent. 



G. The A-Protein 



Treatment with alkali is also used to produce native virus protein or "A"- 

 protein for experimental work (Schramm et al., 1955b). The A-protein is 

 actually a mixture of substances in considerable flux. The smallest fragments 



