THE SYNTHESIS OF PROTEINS H 



to RNA would be a direct synthesis of ribosomal RNA on DNA in the 

 nucleus in a manner analogous to the duplication of DNA itself. 



The second step, the transfer of the genetic information of ribo- 

 somal RNA to protein, is the phenomenon I should like to focus upon, 

 however. Amino acids must be activated before they are able to con- 

 dense to form peptides, and it is now well established that this event is 

 catalyzed in living systems by a group of amino-acid-activating en- 

 zymes, probably one for each of the 20 amino acids. The reaction in- 

 volves the formation, from ATP and amino acid, of an aminoacyl 

 adenylate compound, firmly bound to its specific activating enzyme, as 

 shown in Figure 1. It is highly probable that this is the first step in 

 protein biosynthesis. 







II II 



RCHC-0~ +E+ATP^E(AMP-0-CCHR)+PP 

 NH3 NH3 



Figure 1. 



These aminoacyl adenylate-enzyme complexes then participate in 

 a reaction with a particular kind of cellular RNA, called transfer RNA 

 (or soluble RNA), to bring about attachment of the activated amino 

 acid to the RNA. This reaction may be formulated as shown in Figure 2. 







II II 



E(AMP-0-CCHR)+ sRNA^^sRNA-CCHR+E + AMP 

 NH3 NHJ 



Figure 2. 



During the past three years much of the detail of this presumed 

 second step in protein synthesis has been brought to light. Transfer 

 RNA appears to be a heterogeneous collection of RNA species, of mo- 

 lecular weight 15,000-40,000, each specific for a particular amino acid. 

 The amino acids are linked to the terminal nucleotide residues of each 

 RNA by esterification on the 2' or 3' ribose hydroxyl group. This link- 

 age is of the "high-energy" type, since the two reactions by which the 

 ester is formed are reversible ( Figures 1 and 2 ) . Thus the amino acids 

 remain in the activated state. 



It is known that the three terminal nucleotide residues of the 

 transfer-RNA molecules are cytidylic acid-cytidylic acid-adenylic acid, 



