BIOCHEMICAL MECHANISMS 47 



RNA has been repeatedly confirmed since Hoagland et al.'s (1957) 

 initial observations. There is growing evidence for the view that this 

 soluble fraction is, in fact, a mixture of many different specific 

 RNA's. Each of them would be a specific acceptor for a definite 

 amino acid. 



The role of soluble RNA in protein synthesis and the great sen- 

 sitivity of the reaction to ribonuclease offer an explanation of the 

 results presented in section 6 (p. 30) on the effects of this enzyme 

 on living cells. As already pointed out, in onion roots at least, treat- 

 ment of the living cells with ribonuclease leads to a rapid and con- 

 siderable (50%) decrease in the soluble RNA content, without 

 affecting the RNA present in cell particles (Brachet and Six, 1959). 

 It would certainly be interesting to extend these observations to 

 other cells in which ribonuclease stops protein synthesis in vitro. 



The mechanism of protein synthesis in three different steps (acti- 

 vation of the amino acid by ATP and the soluble enzymes, incor- 

 poration of the amino acid into soluble RNA, and incorporation 

 of the latter into microsomal RNA) which has been postulated by 

 Hoagland et al. (1957) is certainly basically correct. But, for the 

 biologist, and especially for the geneticist and the immunologist, 

 the major problem to be solved is the mechanism of specific pro- 

 tein synthesis. This problem is still at the stage of ingenious hypo- 

 theses. The major one, for which there is no satisfactory substitute 

 so far, is the so-called template hypothesis, which postulates the 

 existence of a model (template) under the influence of which the 

 building blocks (the amino acids) are arranged in the right order. 

 The template would act as a mold forming a counterpart to the 

 protein to be formed. It is tempting to suppose, as many have 

 already done (Friedrich-Freksa, 1940; Rondoni, 1940; Haurowitz, 

 1949, 1952 and Caldwell and Hinshelwood, 1950), that it is RNA 

 which represents the counterpart to the protein. More recently, this 

 view has also been accepted by Borsook (1956a,b) and an impres- 

 sive case has been made in its favor by Spiegelman et ah (1955) and 

 by Spiegelman (1956). 



The main argument in favor of the template theory is the fact 

 that in many instances protein synthesis occurs directly 



References p. 50/54 x v > 



