34 THE BIOSYNTHESIS OF PROTEINS 



spiral has nothing in common with the double helix structure of DNA, 

 the spires of the RNA strands are far apart and separated by protein 

 material. 



For virus RNA as for DNA, it is reasonable to assume that whatever 

 genetic information the linear chain of RNA can carry must be recorded in 

 the molecule as a particular arrangement of the purines and pyrimidines. 

 This idea is supported by experimental facts. For instance, a very mild 

 treatment of the virus by nitrous acid causes the appearance of mutants 

 (Gierer and Mundry, 1958; Boeye, 1959; Schafer, 1959; Mundry, 1959) 

 and it would seem that the appearance of a given mutant results from 

 deamination of a single base (Gierer and Mundry, 1958; Boeye, 1959). 



Tsugita and Fraenkel-Conrat (1960) have isolated a mutant of tobacco 

 mosaic virus which had been obtained after treatment of the isolated virus 

 RNA by nitrous acid. The amino acid composition of the protein coat of 

 this mutant seems to differ from that of the original wild strain by the re- 

 placement of three amino acid residues — one proline, one aspartic acid 

 and one threonine — by one residue each of leucine, alanine and serine. 

 Moreover, the amino acid sequence of the last 17 amino acids of the N- 

 terminal end of the protein has been established for both the original virus 

 and the mutant. They are shown hereafter: 



parent original strain: 

 -Ser-Ser-Phe-Glu-Ser-Ser-Ser-Gly-Leu-Val-Try-Thr-Ser-Gly-Pro- 

 Ala-Thr-OH 



mutant : 

 -Ser-Ser-Phe-Glu-Ser-Ser-Ser-Gly-Leu-Val-Try-Thr-Ser-Gly-Lez<- 

 Ala-Thr-OH 



Fig. 18. 



In the mutant, one proline has been replaced by one leucine, the rest of the 

 sequence remaining unchanged. It would seem that deamination of a base 

 in the virus RNA has caused the substitution of one amino acid for another 

 at a specified point within the polypeptide sequence. This is strikingly 

 similar to the modifications caused in human haemoglobin by spontaneous 

 mutations. Just as human genetic material contains information relevant 

 to the amino acid sequence of haemoglobin, virus RNA contains informa- 

 tion which controls the amino acid sequence in viral protein. This informa- 

 tion is modified when an amino group of certain bases within the RNA 

 is replaced by a keto group. 



Both DNA and RNA can be visualized as long strands of undifferentiated 

 material, the phosphate-carbohydrate backbone, upon which purines and 

 pyrimidines of a few different types (usually 4 different types) are fixed 

 at regular intervals. The sequence of these constitutes genetic information, 



