186 



S. S. COHEN 



mechanism for specific replication exists for RNA, at least as well as for 

 DNA, is of course affirmed by the specific activities of the RNA of tobacco 

 mosaic virus and its strains in initiating viral infection, although, as in the 

 case of DNA, it is not yet proved that nucleic acid replicates itself without a 

 non-nucleic acid intermediary. A few other instances of specificity of RNA 

 and RNA-like structures had been observed before the recent work with 

 virus RNA. For example, it had been shown that certain bacterial RNAs 

 inhibited a bacterial deoxyribonuclease, which was not inhibited by RNA 

 from the mammal, plant, or yeast (Bernheimer, 1953). In addition, poly- 

 rib ophosphate, a double chain of polymerized ribose phosphate linked 

 Ci — — Ci, thereby eliminating purines and pyrimidines, has been recog- 

 nized as the type-specific substance oi Hemophilus influenzae, type B (Zamen- 

 hof et al, 1953). 



2. The Biosynthesis of RNA 



a. Transesterification with Ribonuclease. Digestion of RNA with pancreatic 

 ribonuclease results in the splitting of aU internucleotide linkages which 

 involve C3' of a pyrimidine nucleotide. The first step appears to involve the 

 formation of cyclic nucleotides, i.e., pyrimidine nucleoside 2' : 3' phosphates 

 and the simultaneous cleavage of the linkage to the 5' hydroxyl of the next 

 nucleotide. The cyclic phosphates then hydrolyze to form the 3' nucleotide, 

 as represented in formula (XXXII). 



Pyrimidine 



O — CH 



Pyrimidine 



Pyrimidine 



C- 



C O 



r, Ribonuclease J. ^ 

 K »- L U 



1 



P OH 



Ribonuclease 



O 



C — OH 



C — O-^POjHz 



C— 



\ 



O— CH, 



O— CHj 



(XXXII) 



The first reaction is thus seen to involve a transfer of phosphate from the 

 C5' hydroxyl of one nucleotide to the Cg' hydroxyl of the pyrimidine nucleo- 

 tide. In the second hydrolytic reaction, transfer is made to water. Various 

 inhibitors, such as heparin, appear to affect the second reaction. A spleen 

 ribonuclease or phosphodiesterase produces this end result without the inter- 

 mediation of the cyclic nucleotide. 



Both enzymes have now been demonstrated to participate in synthetic 

 and exchange type reactions. Ribonuclease can catalyze the formation of 

 esters of 3' nucleotides from cyclic nucleotides and alcohols. Spleen phospho- 



