37. NUCLEIC ACID AND PROTEIN SYNTHESIS 



381 



ENZYME, + F^COOH + PR 



Pj 



?=^ ENZYME, R,COO N 



RJ 



+ PP (4) 



ENZYME, R,COO N 



X Y C C A 



Pj 



XYCCA hOH 



K P 



K R 



>|'N NN N 



R 



R 



-OOCR, + ENZYME ' + 



r \l 



Fig. 3. Amino acid attachment to transfer RNA. 



(5) 



linking of the amino acid to the RNA. Widespread study of this phenome- 

 non in many laboratories has led to the general acceptance of the formu- 

 lation of the reaction as depicted in Fig. 3. 



This formulation states that an amino acid (Ri) is first activated by 

 its specific activating enzymei, through formation of an enzyme-bound 

 amino acyl adenylate compound. This compound, with associated acti- 

 vating enzyme, then reacts with an RNA molecule specific for the par- 

 ticular amino acyl adenylate-enzyme complex, resulting in the esterifi- 

 cation of the amino acid on the 2'- or 3'-hydroxyl of the terminal adenosine 

 of the RNA, and the release of the adenylate moiety of the activated 

 compound. The formulation further states that since, as we have seen, 

 the terminal configuration of nucleotides pCpCpA is common to all RNA's 

 involved in the process, the specificity of attachment must be determined 

 by features of the RNA molecule internal to this terminal grouping. (This 

 is indicated in the figure by the subscript number 1 after the nucleotide 

 sequence of the RNA). 



The evidence which supports this formulation must now be examined. 



(1) Amino Acid Activation as a Prelude to Attachment of Amino Acid. 

 Activation of amino acids by the mechanism depicted in Eq. (4) in Fig. 

 3 is now well established as the principle pathway of activation of amino 

 acids in all tissues thus far examined. The discovery that animal tissues 

 catalyzed an amino acid dependent incorporation of PP into ATP, and 

 an ATP dependent formation of amino acid hydroxamic acids, 113-115 pointed 

 to the likelihood that amino acids were activated by formation of such 

 amino acyl-AMP compounds. The fact that these compounds were firmly 

 bound to enzymes was suggested by the finding that AMP failed to ex- 

 change with ATP in the system, and that no acid soluble product was 



