31. SYNTHESIS OF POLYNUCLEOTIDES 135 



crosomal and nuclear RNA are inert. The enzymes responsible for the 

 terminal additions are also present in these soluble fractions (designated 

 pH 5 fraction by these authors). 74 ' 78 By incubations with labeled CTP and 

 ATP, the additions have been shown to occur in a definite sequence. With 

 CTP alone two cytidine-5'-phosphoryl residues may be added to the 3'- 

 hydroxyl end of the chains and the final addition is that of a single adeno- 

 sine-5'-phosphoryl residue. The last addition can only occur when the 

 cytidylyl residues are present. This is in agreement with the earlier studies 

 described above. Labeled UTP when incubated in the absence of other 

 nucleoside triphosphates is also capable of adding a uridine-5'-phosphoryl 

 residue to the ends of polynucleotide chains. 



The terminal additions are magnesium-dependent and are inhibited by 

 inorganic pyrophosphate. In fact, the above additions have been shown to 

 be reversed by inorganic pyrophosphate. 



It seems probable that the RNA-dependent incorporation and the ex- 

 change reactions of ribonucleoside-5 '-triphosphates in extracts of embryonic 

 chicken livers 79 are another example of the pyrophosphorolysis of the end 

 groups of soluble RNA (see, however, ref. 79 a ). 



Hurwitz 80 et al. have reported on the purification of an enzyme from calf 

 thymus nuclei which is specific for transferring a cytidine-o'-phosphoryl 

 group from CTP to the ends of ribonucleic acid chains. The requirement 

 for RNA is highly specific in that only that from thymus gland serves as the 

 acceptor and additions can occur to chains bearing all of the four different 

 ribonucleosides at the ends. Since the enzyme is also highly specific for 

 CTP it would appear that different enzymes are required for additions of 

 different nucleotides to the ends. 



Hecht et al. 77 and a number of other workers 81, 82 have shown that the end 

 unit containing adenosine nucleotide provides a functional grouping in 

 the soluble RNA 76 which is required for its action as a carrier of activated 

 amino acids. 



78 E. B. Keller and P. C. Zamecnik, J. Biol. Chem. 221, 45 (1956). 



79 C. W. Chung and H. R. Mahler, J. Am. Chem. Soc. 80, 3165 (1958). 



79a Added in proof: Chung and Mahler [Biochem. Biophys. Research Communs. I, 232 

 (1959)] have presented further evidence showing that soluble enzyme preparations 

 from cytoplasmic fractions of chick embryo hearts or livers are capable of incorpo- 

 rating adenosine-5'-phosphoryl portion of ATP-C 14 into RNA in nonterminal po- 

 sitions. 



80 J. Hurwitz, A. Bresler, and A. Kaye, Biochem. Biophys. Research Communs. 1, 3 

 (1959). 



81 H. G. Zachau, G. Acs, and F. Lipmann, Proc. Natl. Acad. Sci. U. S. 44, 885 (1958). 



82 J. Preiss, P. Berg, E. J. Ofengand, F. H. Bergmann, and M. Dreckmann, Proc. Natl. 

 Acad. Sci. U. S. 45, 319 (1959) . 



