284 NUCLEIC ACIDS AND GROWTH 3 



recently been reported by Chevremont and Chevremont-Comhaire (1955) on tissue 

 cviltures of fibroblasts : it had already been found by Chevremont and Firket ( 1 952) 

 that RNA stimulates the growth of slow-growing tissue cultures. In their recent 

 experiments, Chevremont and Chevremont-Comhaire (1955) observed an in- 

 hibition of growth in cultures treated with ribonuclease (1/300-1/1,000); as with 

 the amoebae, addition of RNA restores the growth capacity of the ribonuclease 

 treated cells. An interesting point in these investigations is that, according to 

 quantitative cytochemical observations, ribonuclease induces a drop in the RNA 

 content of the cytoplasm and the nucleoli, but that it does not interfere with DNA 

 synthesis : the cells are blocked in early prophase and the DNA content of their 

 nuclei is 4 times that found for spermatozoa. 



Finally, valuable results have also been obtained with ribonuclease in the case 

 of two RNA-containing viruses: tobacco mosaic virus (Casterman and Jeener, 

 1955) and influenza virus (Le Clerc, 1956). In both cases, multiplication is inhi- 

 bited when the host cells (tobacco leaves and chorioallantoic membrane, respec- 

 tively) are treated with ribonuclease prior or shortly after infection. Such findings 

 lend additional support to Jeener's hypothesis on the genetic role of RNA : ap- 

 parently, ribonuclease interferes with the virus multiplication at a stage of its 

 development when RNA and protein are being synthesized independently. 



To summarize, ribonuclease can penetrate in a number of living cells, without 

 interfering with the energy producing mechanisms; the enzyme deeply modifies 

 RNA metabolism. In all cases so far studied, growth, mitotic activity, protein 

 synthesis and incorporation of amino acids into proteins are drastically inhibited. 

 Addition of RNA, in many instances, alleviates to a certain extent the effects of 

 ribonuclease. The experimental facts, which require further and deeper analysis, 

 strongly indicate that RNA integrity is essential for growth and protein synthesis. 



(d) The mechanisms of protein synthesis 



It is beyond the scope of this chapter to discuss at length the many hypotheses 

 which have been proposed to explain protein synthesis; a very short summary of 

 what appears to be the most likely hypothesis at present will be given and the reader 

 is referred for further details to two recent reviews (Brachet, 1955a; Borsook, 1955). 



The main problem, for the biologist, is the mechanism of formation of specific 

 proteins; only the so-called template hypothesis, for which there is no substitute so 

 far, can achieve this aim. The hypothesis postulates the existence of a model 

 (template) under the influence of which the building blocks are arranged in the 

 right order. The template would form a mould, constituting the counterpart of 

 the protein to be formed. 



In most of the recent versions of the template hypothesis, the template is identified 

 with RNA and the building blocks with free amino-acids. According to the hypo- 

 thesis, a great number of specific RNA's exist, each one being the counterpart of 

 the specific protein which is to be synthesized. 



The reasons why the hypothetical template might be identical with RNA are 

 obviously based on the belief that RNA is essential for protein synthesis ; there is, 

 of course, no point in discussing them further now. The best available facts in 

 favour of the identity of RNA with the template lay in Gale and Folkes' (1955b) 



