512 J. BRACKET 



Finally, the fact mentioned earlier in this report that incorporation of 

 thiouracil into the PNA of virus-diseased plants decreases markedly and 

 simultaneously the turnover rates of normal and virus proteins, as well as 

 those of the purines and phosphorus in normal and virus PNA ( Jeener^"^) , 

 is obviously in excellent agreement with the template hypothesis; altera- 

 tions in the chemical constitution of the PNA model or organizer should 

 lead to disturbances in protein synthesis. 



It is possible that the microsomes, w^hich are rich in PNA and incorporate 

 amino acids very actively, contain the templates on which the proteins 

 are synthesized; but whether the building blocks for protein synthesis are 

 free amino acids or intermediates such as polypeptides, is still open to 

 discussion. Genetic evidence suggests that enzyme synthesis requires the 

 presence of free amino acids; if intermediates exist, they do not accumulate 

 (Bonner^^-). Similar conclusions can be drawn from work by Halverson 

 and Spiegelman-^2 on the inhibition of enzyme formation by amino acid 

 analogues: "free amino acid metabolism constitutes a major component of 

 the enzymes synthesizing mechanism." Similar implications arise from 

 Hokin's-^^ work, which show^s that the synthesis of amylase by pancreas 

 slices is markedly increased by the addition of the essential amino acids 

 present in the enzyme molecule. 



On the other hand, isotope experiments by Anfinsen and Steinberg^'*"^^^ 

 on the syntheses of ovalbumin in minces of surviving oviduct and of ribo- 

 nuclease in pancreas slices show that the specific activities of the labeled 

 amino acid (alanine for ovalbumin, phenylalanine for ribonuclease) vary 

 in different parts of the molecule; these findings strongly suggest that 

 peptides are intermediates in protein synthesis, a conclusion also accepted 

 by Campbell and Work-'^ in studies on the biosynthesis of milk proteins. 



Recent work by Peters-'^ and by Koritz and Chantrenne*^* has further 

 shown that incorporation of labeled amino acids precedes the appearance 

 of specific labeled proteins: the conversion of amino acids into protein 

 requires time, but, as Peters points out, the results are compatible both 

 wdth the intermediate or peptide theory and with the template hypothesis. 



But, whatever the real mechanism may be, there is no doubt that pro- 

 tein synthesis in vitro, as well as the uptake of amino acids, requires energy: 

 whether this energy is required for initial peptide synthesis only or for the 

 building up of complete specific proteins on a template as well is not known. 



212 H. O. Halvorson and S. Spiegelman, /. Bacteriol 64, 207 (1952). 



213 L. E. Hokin, Biochem. J. 50, 216 (1951). 



21^ C. B. Anfinsen and D. Steinberg, J. Biol. Chem. 189, 739 (1951). 

 216 D. Steinberg and C. B. Anfinsen, Federation Proc. 11, 292 (1952). 



216 D. Steinberg and C. B. Anfinsen, J. Biol. Chem. 199, 25 (1952). 



217 P. N. Campbell and T. S. Work, Biochem. J. 52, 217 (1952). 



218 T. Peters, Jr., J. Biol. Chem. 200, 461 (1953). 



