102 J. N. DAVIDSON 



Discussion 



DiscHE : You indicated in a slide that the deoxycytidine kinases did not increase 

 in regenerating liver but that the thymidine kinase strongly increased; is that 

 because the original activity of the deoxycytidine kinase in the liver was so much 

 higher than that of the thymidine kinase ? 



Davidson : Yes, the activity of deoxycytidine kinase in the liver is much higher 

 than that of thymidine kinase and the same holds for the kinases for deoxyAMP, 

 and deoxyGMP; and this may well be the reason why they do not increase because 

 they are abundant already. 



Reichard : I should like to ask you two questions with regard to the inhibition 

 of the syntheses ; one is : have you tried to fortify the system by adding an ATP- 

 regenerating system which might overcome this inhibition ? The second question 

 is : does the factor which inhibits the polymerase have any deoxyribonuclease 

 activity ? 



Davidson: We have investigated both of these points. We haven't added an 

 ATP-regenerating system, we have infused ATP into the system without effect. 

 We have looked into the question of deoxyribonuclease activity and as far as we 

 can see this has nothing to do with the action of the inhibitory factor. 



SiEKEViTZ : I should like to ask Prof. Davidson a couple of questions. Another 

 possibility of regulation of DNA synthesis derives from the possibility that DNA 

 itself may be a cofactor for the enzyme. Could it be that the availability of the right 

 kind of DNA which acts as a primer could act as a regulator of DNA synthesis ? 

 In the resting cell DNA could not act as a primer for the enzyme but at a certain 

 point something happens, it can act as a primer, and when finished it "rejuvenates" 

 itself until it is no longer active. 



Davidson : Offhand I am not very convinced about it, but I should point out 

 that conditions in the resting cell are very different from those in a cell-free 

 system such as ours. 



Reichard : If I understood Dr. Siekevitz right he implies that a specific DNA 

 must be present in the system in order to start off DNA synthesis. Romberg's 

 data would argue strongly against this, since one can use any DNA as a primer for 

 a polymerase from E. coli. 



Siekevitz: Is that single strand or double strand DNA ? 



Reichard: That's a different question again, which is not cleared up. Even in 

 our crude preparations, extracts of chick embryos, we get very little DNA synthesis 

 when we add highly polymerized double-stranded calf thymus DNA. As soon as 

 we heat this preparation for lo min., this treatment is supposed to split the double 

 strands, the DNA will act as a primer. 



Siekevitz: But that might be just the point. There comes a time in the life of 

 the cell when single strand DNA is formed to act as a primer, and this splitting of 

 the RNA might be the point where regulation could occur. 



Reichard : I think that is quite true and that it might be an important regu- 

 lating mechanism in living cells. 



Davidson : I think I should perhaps mention that in the experiments which 

 I described we use heated DNA as our primer but, of course, this is a cell-free 

 system. 



