BIOSYNTHESIS OF PROTEINS 279 



occupy a very prominent place among these systems in deter- 

 mining the specific structure of any particular protein. In 

 Gale's experiments the addition of nucleic acid to structural 

 fragments of bacterial cells restored their ability to synthesise 

 particular proteins. Similarly, the introduction of the RNA 

 of tobacco mosaic virus into the leaves of tobacco plants 

 creates suitable conditions for the synthesis there by the 

 protoplasmic systems of the cell of the specific X-protein 

 which could not have been formed there before the infection 

 and which seems to be the result of distortions of the process 

 of biosvnthesis of protein, distortions determined by the 

 intramolecular structure of the viral nucleic acid. 



The specificity of the viral RNA compared with other 

 similar nucleic acids consists simply in the fact that the viral 

 RNA can enter actively into the metabolism of the tobacco 

 plant and, to some extent, overcome the influence of the 

 nucleic acids of the plant itself and enable the leaf to syn- 

 thesise proteins which are foreign to it and which accumulate 

 there in very large and, therefore, easily detectable amounts. 

 It may be that more careful study would show that many other 

 nucleic acids could also alter, to some extent, the course of 

 biosynthesis in foreign organisms into which they were intro- 

 duced in the native state. In particular we are convinced 

 that the experiments, which have recently become widely 

 known, in which one strain of bacteria is transformed into 

 another under the influence of DNA prepared from the 

 latter, are cases in point.''"' 



An understanding of the part played by the intramolecular 

 structure of nucleic acids in the biosynthesis of proteins with 

 specific structures was made more difficult until very recently 

 by the over-simplified hypotheses concerning the structure of 

 the nucleic acids themselves. 



It is still not long since it was accepted, in accordance with 

 the results of P. A. Levene and R. S. Tipson"' that the 

 fundamental units of RNA and DNA consisted of tetra- 

 nucleotides, i.e. complexes composed of four appropriate 

 mononucleotides united w^ith one another. According to this 

 hypothesis the tetranucleotide complex of RNA is made up 

 of adenine nucleotide, guanine nucleotide, cytosine nucleo- 

 tide and uracil nucleotide while DNA is composed of adenine 



