IV RNA AND GROWTH PROTEIN SYNTHESIS 285 



observations, which have already been mentioned (p. 278) : as will be remembered, 

 they have definitely proved that small fragments (oligonucleotides), obtained by 

 ribonuclease digestion of RNA, promote the incorporation of different amino acids 

 in a relatively specific way. It has been suggested by Gale (1955) that "the factors 

 isolated from ribonuclease digests represent specific groupings forming loci for the 

 combination of specific amino acids, and bringing about exchange of those amino 

 acids with corresponding residues in proteins of the preparation". Such an explana- 

 tion might very well also be valid in the cases we have studied, where addition of 

 RNA restored amino-acid incorporation in ribonuclease-treated living amoebae. 



Recent work on microorganisms, especially the experiments of Spiegelman, 

 Halvorson and Ben-Ishai (1955), provides strong evidence in favour of the view 

 that free amino acids are the building blocks for protein synthesis: induced en- 

 zyme synthesis occurs de novo, directly at the expense of the free amino-acid pool 

 of the cell; there is no indication for peptides intermediates in such cases. Most of 

 the work done on animal tissues supports this contention [for instance that of 

 Heimberg and Velick (1954) and of Askonas et al., (1955)]; however, experiments 

 by Anfinsen and Steinberg (1951,1952) on unequal labelling of proteins synthe- 

 sized in vitro by tissue slices incubated in the presence of ^"^002 are difficult to recon- 

 cile with this view. Their results might, of course, be due to the unphysiological 

 in vitro conditions they adopted. On the whole, present evidence is very much in 

 favour of the identification of the building blocks with the free amino acids. 



The mere presence of RNA and amino acids is not, however, enough to synthe- 

 size proteins : as pointed out repeatedly by many biochemists, especially Borsook 

 (1955), Lipmann (1949), Chantrenne (1953) and Siekevitz (1952), an energy 

 source, presumably ATP, should be available to the template. How and when 

 this energy source comes into the picture is not clear, and only hypotheses can be 

 made: one possibility, which has been proposed by Chantrenne (1953) and dis- 

 cussed recently in detail by Borsook (1955), is that amino acids are activated prior 

 to their fixation on the template. This activation might well be a phosphorylation 

 at the expense of ATP, since we know from Chantrenne's (1953) work that syn- 

 thetic phosphorylated amino acids easily react with ordinary amino acids to 

 form peptides. Another possibility, which has been suggested by Bounce (1952), 

 is that the template itself, i.e. RNA, is phosphorylated and activated by ATP : the 

 usual free amino acids would attach themselves on the activated template, their 

 position being determined by specific nucleotide arrangements in the RNA mole- 

 cule. Further discussion of this question, in view of the scantiness of experimental 

 evidence, seems to be fruitless for the time being^ 



V. NUCLEIC ACIDS AND EMBRYONIC DEVELOPMENT 



{a) Synthesis of DNA and RNA during development 



Early work by Brachet (1933) has shown that unfertilized eggs always con- 

 tain a store of RNA and that a marked DNA synthesis occurs during development; 



1 More recent developments in this field will be found in a paper by Hoagland, Zamecnik 

 and Stephenson (1957). 



Literature p. zgg 



