380 



u 



MAHLON B. HOAGLAND 

 X Y C 



X Y C 



+ PP 



^Pm 



+ PP 



(1) 



X Y C C 



+ PR P 



X Y C C 



•J 'Nl Nl Nl 



+ PP 



(2) 



X Y C C 



X Y C C A 



\ hP 



\l Ni N 



+ PP 



+ PP (3) 



Fig. 2. Terminal nucleotide additions to transfer RNA. 



of Hecht et al. using a cruder system, and therefore perhaps more rep- 

 resentative of in vivo events. Furthermore, the RNA used in this system 

 had had the terminal groups removed by preincubation.) These studies 

 do not, however, rule out the unlikely possibility that the innermost CMP 

 is deeper in the polynucleotide chain. (2) When ATP is present in this 

 incubation with the CTP-C 14 , essentially all of the labeled cytidine is 

 recovered as the 2',3'-cytidylic acid. (3) Incubation with ATP-C 14 essen- 

 tially leads to complete terminal attachment of the AMP-C 14 . (4) PP 

 inhibits the labeling of RNA by both CTP-C 14 and ATP-C 14 ; PP 32 is in- 

 corporated into CTP in the presence of RNA and the appropriate enzymes 

 and PP is the product of AMP terminal attachment from ATP. (5) The 

 reversibility of the reactions is further supported by the fact that the 

 enzymic loss of the terminal groups is dependent upon PP, provided 

 measures are taken to remove PP from the enzyme preparation. (6) The 

 addition of other nucleoside triphosphates has no effect on the pattern 

 of labeling obtained with ATP and CTP. (7) Naturally occurring sRNA 

 has only adenosine and cytosine ends upon alkaline hydrolysis. 158 



The foregoing seems to be the mechanism by which transfer RNA is 

 prepared for amino acid attachment; although each molecule would ap- 

 pear to consist of a different series of bases, the common terminal con- 

 figuration to which the amino acid is to be attached has been prepared. 

 In the next section we shall examine the mechanism of this amino acid- 

 transfer RNA interaction. 



c. Enzymic Attachment of Amino Acids to Transfer RNA 



Incubation of naturally occurring sRNA, amino acid activating enzymes, 

 ATP, Mg ++ , and the appropriate C 14 -amino acids results in the covalent 

 158 D. B. Dunn, unpublished data, (1958). 



