STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 



187 



diesterase can exchange one alcohol for another on such a phosphate ester 

 (Heppel and Whitfield, 1955). 



When ribonuclease is incubated with cytidine-2' 3 '-phosphate under suit- 

 able conditions, polynucleotides are formed, made exclusively of cytidylic 

 acid linked through 3' and 5' hydroxyls. CycHc cytidine nucleotide could also 

 be linked in this way to adenosme or cyclic adenosine nucleotide. The other 

 cyclic pyrimidine nucleotide, uridine-2 '3 '-phosphate, could also be used as a 

 donor in synthetic reactions with ribonuclease. The acceptor in these reac- 

 tions must be a primary alcohol. 



Although spleen phosphodiesterase was inactive with cyclic nucleotides, it 

 could transfer nucleotides, as in the reaction: 



adenylyl uridylic acid + cytidine — — -> adenylyl cytidine + uridylic acid 

 When benzyl esters of mononucleotides were incubated with the enzyme, 

 monobenzyl esters of dinucleotides were formed. These synthetic reactions 

 and their possible significance have been summarized and discussed by 

 Heppel et al. (1955). 



Crystalline pancreatic ribonuclease has now been separated into two 

 fractions, one of which catalyzes a reversible exchange or transesterification 

 between cycHc pyrimidine nucleotides and nucleosides. The other fraction 

 irreversibly synthesized dinucleoside monophosphates from cyclic nucleotides 

 and a nucleoside (Hakim, 1957). 



6. Polynucleotide phosphorylase. The problem of the mechanism of RNA 

 synthesis was signally advanced by the discovery of a polynucleotide phos- 

 phorylase in bacteria, e.g., Azotohacter vinelandii, by Grunberg-Manago and 

 Ochoa (1955). The enzyme catalyzes the reversible synthesis of polyribonu- 

 cleotides from 5 '-nucleoside diphosphates, releasing inorganic ortho P in the 

 process: 



n XPP ^ (XP)„ -f JiP 

 The enzyme may be assayed by observing the exchange of P^^ with the 

 termmal P of nucleoside diphosphates. 



Base 



Base 



Base 



C- 



COH 

 n 



COH 



C 



o I 



II II I 



HO-P-O-P-O-CH, 



1 I '■ 

 OH OH 



/7H3PO4 



O-CH, 



