378 MAHLON B. HOAGLAND 



hydrolysis. In agreement withHeidelbergerd a/., 149 this AMP appeared to be 

 chiefly incorporated adjacent to CMP. Several of these studies also indi- 

 cated that the immediate precursor of the end group was the triphosphate. 



Detailed analyses of these terminal nucleotide addition reactions have 

 been carried out by Hecht et a/., 122 ' 155 and by Herbert. 123154 



It is now established that the AMP and cytidine monophosphate moie- 

 ties of ATP and cytidine triphosphate (CTP) are attached in a specific 

 manner to the ends of the transfer RNA molecules by a specific enzyme 

 system found in the soluble fraction of the cell. The experimental evidence 

 may be conveniently considered under the following headings: the identity 

 of the RNA acceptor, the nucleotide precursors of the end groups, the en- 

 zymes catalyzing the reactions, and the mechanism of the reactions. (See 

 Figs. 2 and 3 for the formulas of the reactions discussed under these head- 

 ings.) 



(2) The Identity of RNA Acceptors of the End Groups. It has been estab- 

 lished that the RXA which participates in the reactions involving the 

 attachment of nucleotide end groups is the soluble RXA of the cell, sRNA. 

 RXA derived from the nucleus and from the ribosomes are essentially 

 inactive in the system. sRNA fractions from widely different species, how- 

 ever, are interchangeable in the system; yeast, bacterial, and mammalian 

 sRNAs are readily labeled with terminal nucleotides in the presence of 

 ascites tumor or rat liver enzymes. It can be stated furthermore, that 

 transfer RNA itself is the acceptor of the terminal grouping of nucleo- 

 tides for it has been shown that the latter reaction is an obligatory prelude 

 to amino acid attachment. 



(3) The Nucleotide Precursors of the End Groups. The triphosphates of 

 adenosine, cytidine, and uridine serve as the source of the respective 

 mononucleotide end groups of transfer RNA. The terminal attachment 

 of adenosine and cytidine triphosphates exceeds that of the uridine com- 

 pound by at least a factor of 4. 122 The significance of the latter reaction 

 (that involving UTP) has yet to be assessed. GTP does not seem to par- 

 ticipate significantly in these reactions. In view of the reactions described 

 by Grunberg-Manago et aL, 156 and Hilmoe and Heppel 157 in which nucle- 

 oside diphosphates served as the substrates for RNA synthesis in bacterial 

 systems, it was important to know what level of phosphorylation of nu- 

 cleoside was involved in the terminal attachment reactions. The triphos- 

 phates are clearly superior to the diphosphates, however, in these reactions. 

 Furthermore, evidence has been presented by Hecht et al., v ~- 155 that the 



155 L. I. Hecht, M. L. Stephenson, and P. C. Zamecnik, Pror. Natl. Acad. Set. U. S. 

 45, 505 (1959). 



156 M. Grunberg-Manago, P. J. Ortiz, and S. Ochoa, Science 122, 907 (1955). 



157 R. S. Hilmoe and L. A. Heppel, «/. Am. Chem. Soc. 79, 4810 (1957). 



