STUDIES ON MECHANISM OF SYNTHESIS OF SOLUBLE RIBONUCLEIC ACID II 9 



may therefore be concluded that the inorganic pyrophosphate exerts its 

 effect on the S-RXA; in order that this effect be ehcited the concomitant 

 presence of the enzvme is required but not that of the CTP. 



The effect of inorganic pyrophosphate has been elucidated as being 

 one of pvrophosphorolysis of the S-RXA (Table IV) because incubation 

 of the enzyme with S-RNA and p-P]-pyrophosphate results in the 

 liberation of ^-P-labelled ribonucleoside triphosphates into the medium. 



800 



700 X- 



600 



500 



400 



300 - 



100 



"C- AMP-S-RNA 



C-CMP-S-RNA 



1.0 



0.5 



2.0 3.0 4.0 5.0 



Mg /I MOLES PER ML 



6.0 



3.0 



7.0 



3.5 



8.0 



4.0 



.0 1.5 2.0 2.5 



P- P^ MOLES PER ML 



Fig. 5. Pvrophosphorolysis of the terminal ribonucleotides of S-RNA. Two 

 portions of S-RXA-/3 and -y containing 30 F.^eo units each were labelled separately 

 with [C^*]-AMP and with [C'^]-CMP respectively. They were then extracted free 

 of the nbonucleotide-incorporating enz\Tne, and Mg + + and pyrophosphate in the 

 indicated amounts under standard conditions. At the end of the incubation period 

 the residual radioactivity on each type of labelled S-RNA was determined. 



If the converse experiment is performed, that is if ^'-P-labelled S-RNA 

 (obtained after in vivo labelling with ^-P), is incubated with non-radio- 

 active pyrophosphate and enzyme, again ^-P-labelled ribonucleoside tri- 

 phosphates are liberated into the medium. 



Pyrophosphorolysis seems to be directed primarily towards the end of 

 the S-RNA molecule (Fig. 5). This can be shown by using S-RNA which 

 has been labelled in vitro with either [^^C]-ATP or with [^^CJ-CTP which 

 are known to be incorporated in the end of the S-RXA molecule. When 



