134 H. GOBIND KHORANA 



and nucleotide precursors into ribonucleic acids in cell-free systems reported 

 from a number of laboratories 64 " 69 have led to the now widely accepted 

 conclusion that ribonucleoside-5 '-phosphates are intermediates in the bio- 

 synthesis of ribonucleic acids. 



b. Ribonncleoside-5' '-Triphosphates in Additions to Ends of Existing Ribo- 

 polynucleotide Chains 



A reaction which is now relatively well understood as a result of the work 

 in a number of laboratories is the addition of nucleoside-5'-phosphoryl 

 units derived from the corresponding triphosphates to the 3'-hydroxyl ends 

 of preexisting rather low molecular weight ribonucleic acids. The original 

 observation was recorded by Heidelberger et a/. 68 that when adenosine-5'- 

 phosphate labeled with P 32 was incubated with the cytoplasmic fraction of 

 rat liver homogenates, under suitable conditions the nucleotide unit was 

 incorporated intact at the terminus of a ribonucleic acid chain. By alkaline 

 degradation it could be further demonstrated that the addition occurred 

 to the 2'- or 3'-hydroxyl group of a terminal cytidine residue. Subsequent 

 work 70-75 has confirmed this observation and furthermore has shown that 

 ATP is the actual donor of the adenosine-5'-phosphoryl residue for this 

 terminal addition. Further understanding of the nature and significance of 

 this reaction has come from the work of Zamecnik and co-workers. 71, 74> 76, 77 

 The acceptor of the terminal additions was found to be a rather low molec- 

 ular weight RNA present in the high speed supernatant (soluble) fractions 

 of the rat and Ehrlich ascites carcinoma cell homogenates, while the mi- 



64 E. Goldwasser, /. Am. Chem. Soc. 77, 6083 (1955). 



65 V. R. Potter, L. I. Hecht, and E. Herbert, Biochim. el Biophys. Acta. 20, 439 (1956) . 



66 E. Herbert, V. R. Potter, and L. I. Hecht, J. Biol. Chem. 225, 659 (1957). 



67 E. S. Canellakis, Biochim. et Biophys. Acta 23, 217 (1957). 



68 C. Heidelberger, E. Harbers, K. C. Liebman, Y. Takagi, and V. R. Potter, Bio- 

 chim. et Biophys. Acta 20, 445 (1956). 



69 A. R. P. Paterson and G. A. LePage, Cancer Research 17, 409 (1957). 



70 E. S. Canellakis, Biochim. et Biophys. Acta 25, 217 (1957). 



71 P. C. Zamecnik, M. L. Stephenson, J. F. Scott, and M. B. Hoagland, Federation 

 Proc. 16, 275 (1957). 



72 M. Edmonds and R. Abrams, Boichim. et Biophys. Acta 26, 227 (1957). 



73 E. Herbert, J. Biol. Chem. 231, 975 (1958). 



74 L. I. Hecht, P. C. Zamecnik, M. L. Stephenson, and J. F. Scott, J. Biol. Chem. 233, 

 954 (1958). 



76 E. Herbert, Federation Proc. 17, 241 (1958). 



76 M. B. Hoagland, M. L. Stephenson, J. F. Scott, L. I. Hecht, and P. C. Zamencik, 

 /. Biol. Chem. 231, 241 (1958). 



77 L. I. Hecht, M. L. Stephenson, and P. C. Zamecnik, Biochim. et Biophys. Acta 29, 

 460 (1958). 



