38. BIOSYNTHESIS OF PROTEINS IN BACTERIAL CELLS 447 



VI. General Discussion 



The general concepts on protein synthesis, which can be deduced from an 

 examination of the experimental results analyzed above, enable some an- 

 swers to be made to the following questions: 



(a) What do we know about the intermediate steps in amino acid incor- 

 poration into peptide chains? 



(6) What is the nature of, or what are the main components of, the pro- 

 tein-forming system? 



(c) How can we visualize the transfer of genetic information to the pro- 

 tein-forming system? 



(d) How does a cell control its rate of protein synthesis? 



1. Intermediate Steps in Protein Formation 



It has been suggested from the results of in vitro experiments that amino 

 acid activation could be the first biochemical event in the incorporation of 

 amino acids into peptide chains and this hypothesis agrees in certain re- 

 spects with the results of in vivo studies. 



The arguments supporting the occurrence of amino acid activation in 

 vivo during protein synthesis are as follows: 



(1) All the known natural amino acids which compose a protein can be 

 activated. 31 



(2) The steric amino acid analogs, which are known to replace in vivo 

 the corresponding natural amino acids in proteins, 193 " 195 can be activated 

 by those enzymes which are known to activate specifically their natural coun- 

 terpart. This occurs in the case of the tryptophan analogs (azatryptophan, 

 tryptazan), 196 of some phenylalanine analogs (parafluorophenylalanine), 197 

 and of selenomethionine. 198 5-Methyltryptophan which is not incorporated 

 into protein 193 ' 194 cannot be activated. 196 According to Loftfield et al., how- 

 ever, 199 the absence of incorporation may be due, sometimes, to reasons 

 other than the failure in activation. 



In favor of the participation of the sRNA in protein biosynthesis several 

 observations, mostly based on considerations ©f kinetics can be quoted: 



(1) In vivo, radioactive amino acids are incorporated earlier into the 

 sRNA than into the protein. 30 ' 41 



(2) Amino acids bound to the sRNA undergo a very fast renewal during 



193 A. B. Pardee, Biochim. et Biophys. Acta 21, 406 (1956). 



194 R. Munier and G. N. Cohen, Biochim. et Biophys. Acta 21, 592 (1956). 

 196 D. B. Cowie and G. N. Cohen, Biochim. et Biophys. Acta 26, 252 (1957). 



196 N. Sharon and F. Lipmann Federation Proc 16, 246 (1957). 



197 A. Ullmann and F. Gros, unpublished data (1958). 



198 B. Nisman and M. L. Hirsch, Ann. inst. Pasteur 95, 615 (1958). 



199 R. B. Loftfield, L. I. Hecht, and E. A. Eigner, Federation Proc. 18, 276 (1959). 



