. Discussion 305 



very rapidly. I do not have the data with me, but tliere is a measurable 

 concentration of cytidylic acid, a measurable pool, within the cell. 



Racker : For a complete inhibition of the enzyme it would be necessary 

 to reach values at least 5 times that of the Ki value. This would require 

 a concentration of over 20 [jimoles per ml. Do you think that so much 

 accumulates? 



Magasanik : We have measured the levels of the intracellular nucleo- 

 tides in bacteria. Prof. Potter, you could give the answer, because 

 our results were very close to those which you obtained in animal cells. 

 Potter: The cytidylic content is very small, much smaller than the 

 adenylic and the uridylic levels. But one cannot even begin to dis- 

 qualify the validity of the general line of argument in that way. 



Racker: I agree with you. We have been postulating that compart- 

 mentation is contributing to the Pasteur phenomenon in ascites tumour, 

 and we could invoke compartmentation here, too. But since this is a 

 difficult thing to prove, we are better off if we eliminate other possi- 

 bilities first. 



Magasanik: Cytidylic acid is probably not only a nucleic acid pre- 

 cursor but also a coenzyme; therefore, it should accumulate to some 

 extent. 



Pardee: We worried about this at the time we did the work and we 

 came to the conclusion that it was not very important. Incidentally, 

 the concentration of the enzymes measured in terms of maximum rate 

 in vitro was only slightly greater than that required to furnish observed 

 amounts of pyrimidines, and you do not need to have a drastic inhibi- 

 tion to throttle these reactions down. 



Potter : The demonstration of the excretion into the medium of large 

 amounts of material is of great importance. This proves the feedback ; 

 it is clear that this is not mass action, and that it is not effecting it by 

 equilibrium but by direct inhibition of a prior enzyme. The fact that 

 when uridine or cytidine is supplied, the excretion is completely shut 

 off, is a powerful argument. 



Pardee : As regards mass action, I should mention that one of these 

 steps is irreversible according to Lieberman and co-workers (Lieber- 

 man, I., Kornberg, A., and Simms, E. S. (1955). J. biol. Chem., 215, 

 403). Therefore, there cannot be a mass action, a pushing back. In the 

 second place, in mutants lacking an enzyme at any of a number of 

 points, the mechanism still works. 



Potter: Later on we can perhaps devote some time to a definition of 

 what might legitimately be called feedback, and what is simply mass 

 action or competition for substrate. (See p. 354). 



Magasanik : In every case that has been studied so far, it is the very 

 last product of a biosynthetic sequence, such as valine, isoleucine, 

 threonine, histidine or cytidylic acid, which shuts off the first reaction of 

 the sequence. This constitutes impressive evidence for the general 

 importance of this control mechanism. 



I would like to present to you how a complex system involving two 

 cycles can be regulated in a simple manner by these feedback controls 

 in Escherichia coli and related organisms. We have studied the terminal 



