ROLE OF NUCLEIC ACIDS 69 



purines, pyrimidines and their derivatives favours it in some systems 

 (Hancock, 1957; Reiner, 1960). In disrupted Staphylococcus aureus, a 

 supplement of purines or pyrimidines stimulates protein and enzyme 

 formation (Gale and Folkes, 1953; Creaser, 1955; Gale, 1956). A ribonu- 

 clease hydrolysate of RNA also contains substances which stimulate the 

 incorporation of amino acids into the proteins of such systems; but this 

 effect which was first attributed to oligonucleotides (Gale and Folkes, 

 1955) is in fact due to other substances which have not been identified yet 

 (Gale et ah, 1958). Webster observed that the incorporation of amino acid 

 into cytoplasmic particles from pea roots is markedly promoted by a 

 mixture of the four ribonucleosides, whereas desoxyribosides are inactive. 

 A mixture of the four ribonucleoside triphosphates ATP, GTP, CTP and 

 UTP also stimulated amino acid incorporation (Webster and Johnson, 

 1955; Webster, 1957). 



Thus protein synthesis can, in certain systems, be limited by lack of 

 purine, pyrimidine and derivatives. As the latter substances can serve as 

 precursors of RNA, these observations have often been taken as evidence 

 that protein synthesis requires the synthesis of new RNA molecules. 

 Apparent confirmation of this conclusion was found in the inhibition of 

 protein synthesis by purine and pyrimidine analogues, and by the direct 

 observation of an increased incorporation of labelled adenine or uracil into 

 RNA during induced enzyme formation (Gale and Folkes, 1955; Chan- 

 trenne, 1956). It seemed highly probable therefore that the continuous 

 synthesis of some RNA fraction was necessary for protein synthesis (see 

 review by Spiegelman, 1956). Although this possibility is in no way 

 excluded, it is realized at present that other interpretations of the experi- 

 mental facts are equally probable. The fact that RNA precursors are 

 required for some process does not prove that the synthesis of new RNA 

 molecules is required. Many 'RNA precursors' also participate in the 

 metabolism of low molecular weight material. As will be shown in the next 

 chapter, purine and pyrimidine analogues do interfere with protein syn- 

 thesis, but in most cases it is not by preventing RNA synthesis that they do 

 so. The increased incorporation of adenine which accompanies enzyme 

 induction in yeast is a rather involved process, in some way connected with 

 the breakdown of certain RNA fractions (Chantrenne, 1958). The evidence 

 for a synthesis of new RNA molecules linked to protein synthesis therefore 

 is certainly not compelling. But it remains that metabolic processes'hwolving 

 some RNA fractions are most probably involved in protein synthesis. The 

 nature and significance of these processes is at present anyone's guess. The 

 results of recent investigations have indicated that soluble RNA can bind in 

 succession at the end of its polynucleotidic chains two pyrimidine nucleo- 

 tides followed by a purine nucleotide. The attachment of this terminal 

 sequence of three nucleotides is a prerequisite for the fixation of activated 



