INTERMEDIATE REACTIONS IN PROTEIN SYNTHESIS 



165 



Fig. 3. Schematic diagram indi- 

 cating that when ATP is present, 

 the terminal CMP residue of S-RNA 

 is covered up. 



1. CTP + RNA- ^ RNA-pCpC + PP 



"Apo-RNA' 



2. ATP+ RNA-pCpCFi RNA-pCpCpA + PP 



Fi(i. 4. .\\ailahlc sites for amino 

 acid on S-RNA. 



AMINO ACID 



/ ^ 



BASE BASE BASE / BASE 



OH 



OH J 

 OH 



two separate steps involved in the binding of amino acid to S-RNA. These are 

 indicated in figure 5. The first step would prepare the S-RNA with its specific 

 nucleotide end groups, while the second enzymatic step would use this S-RNA 

 containing the end groups as an acceptor for the amino acid. As evidence for a 

 two-step process, we have never been able to demonstrate an amino acid effect 

 on the addition of the nucleotide end groups to the S-RNA, and furthermore, in 

 the ascites system the nucleotide labeling reaction is saturated at extremely low 

 concentrations of ATP, i.e., 0.0005 ^? whereas the ATP requirement to bind the 

 amino acids to S-RNA is in the order of 50 times as much (17, 22). 



What then is the significance of this highly specific S-RNA in the over-all 

 incorporation of amino acids into protein, and where does it fit into the general 

 scheme.'' GTP plays no role in the binding of amino acid to S-RNA, although 

 it is essential to obtain incorporation into protein. An experiment using intact 

 ascites cells gave us some clues as to the rates of incorporation of amino acid 

 into the various RNA and protein fractions (22). The reactions were so rapid it 

 was necessary to slow them down by carrying out the incubation at 20 °C instead 



ADDITION OF NUCLEOTIDE END GROUP 



RNA- + ATP+ CTP ^ RNA-pCpCpA + PP 



NO AMINO ACID EFFECT 



LOW ATP AND CTP REQUIREMENT 



BINDING OF AMINO ACID 



. AMINO ACID + ATP, + RNA-pCpCpA ?^ RNA-pCpCpA- AMINO ACID 

 activated amino acid 



HIGH ATP REQUIREMENT 



Fi(,. 5. Postulated steps in binding of amino acid to S-RNA. 



