106 THE BIOSYNTHESIS OF PROTEINS 



the Other end of these RNA molecules terminate by guanosine 5' phosphate. 

 Harbers and Heidelberger (1959) have separated RNAs having the usual 

 terminal C-C-A sequence from yet another group in which the final 

 sequence is uridyl-uridyl guanosine. It is not known whether the latter can 

 bind amino acids. 



Evidence discussed before indicates that the carboxyl group of the 

 amino acids is bound to soluble RNA in such a way that it is only moder- 

 ately reactive with hydroxylamine. The reactivity of this bond resembles 

 that of an aminoacyl ester more than that of a mixed phosphoric anhydride 

 (Raacke, 1958; Hoagland et ah, 1958; Lipmann et ah, 1959) and binding 

 of the carboxyl of the amino acid somewhere else than on the phosphate 

 residue is therefore indicated. A soluble RNA loaded with amino acids was 

 hydrolysed by ribonuclease and the split products separated by paper 

 electrophoresis. The amino acids were not released, they were found to be 

 associated with adenosine, from which they could be separated by mild 

 alkali. Since the adenosine amino acid compound does not reduce periodate, 

 it must be concluded that the amino acids are bound to an —OH in position 

 2' or 3' of adenosine (Zachau et ah, 1958), for a compound with hydroxyl 

 groups on two consecutive carbons (like 2' and 3' of adenosine) would be 

 oxidized by periodate. 



The amino acid composition of the mixed adenosine-amino acid com- 

 pounds thus isolated from RNA indicated the presence in various amounts 

 of almost all the amino acids (Lipmann et ah, 1959). Preiss et ah (1959) 

 studying soluble RNA from E. coli obtained similar results. They showed 

 that periodate treatment destroys the acceptor capacity of RNA. More- 

 over, if an amino acid residue is linked to the RNA prior to treatment with 

 periodate, the acceptor site specific for the bound amino acid is protected 

 against inactivation while the others are destroyed. This provides an ele- 

 gant demonstration of the high degree of specificity of the acceptor RNA, 

 and confirms that a free glycol structure is necessary for the fixation of all 

 the individual amino acids. 



In the present stage of our knowledge, it seems that there might exist 

 twenty soluble RNAs, each able to accept specifically one single amino 

 acid. This is bound by ester linkage to the —OH in position 2' or 3' of the 

 adenosine residue of the terminal cytidyl-cytidyl adenosine sequence. 



Two important questions must now be asked; how do the specific RNAs 

 pick up the amino acids, and how do they transfer them further? The puri- 

 fication of several activation enzymes, and the possibility of isolating soluble 

 RNAs by the phenol method which most probably destroys all the enzymes 

 present, made it possible to study the requirement for the transfer of 

 activated amino acid from the enzyme to soluble RNA. Apparently nothing 

 else is required beside the pure activation enzyme and the RNA (Schweet 

 et ah, 1958; Preiss et ah, 1959; Wong et ah, 1959). Capacity of a threonine 



