. Discussion 307 



IMP, and thus the formation of AMP at the cost of GMP. This conver- 

 sion is in turn controlled by ATP, which inhibits the action of the 

 GMP-reductase ; consequently, a high intracellular level of adenine 

 nucleotides will prevent the formation of additional AMP from GMP. 

 In addition, derivatives of the purines control synthesis of IMP de novo 

 by inhibiting an early step of the common biosynthetic pathway (Gots, 

 J. S. (1957). J. hiol. Chem., 228, 57). The discovery of these control 

 mechanisms explains why these organisms use exogenously supplied 

 adenine or guanine preferentially to synthesizing these compounds de 

 novo, and why, when supplied with a mixture of adenine and guanine, 

 they use each purine preferentially for the production of the corres- 

 ponding nucleotide component of the nucleic acids. 



In addition to its other functions, AMP (or ATP) is also the donor of 

 an N-C fragment which becomes the N^-Cg portion of the imidazole 

 ring of histidine. The first step in the pathway leading to histidine is the 

 attachment of ribose 5-phosphate to nitrogen-1 of AMP to give PR-AMP. 

 This compound is aminated by glutamine and cleaved to give the histi- 

 dine precursor IGP (imidazoleglycerol phosphate) and the purine 

 precursor AICAR (4-amino 5-imidazole carboxamide ribonucleotide) 

 (Moyed, H. S., and Magasanik, B. (1958). J. Atner. chem. Soc, 79, 4812). 

 This series of reactions constitutes a cycle from IMP via AMP and 

 AICAR back to IMP. The operation of this cycle is controlled by 

 histidine, which inhibits the formation of PR- AMP from AMP (or ATP) 

 and ribose 5-phosphate. The complete cycle will operate just sufficiently 

 to supply the cell with the histidine it requires for the synthesis of 

 protein ; however, by blocking the path leading from AMP to AICAR, 

 an excess of exogenously supplied histidine would also prevent the 

 conversion of AMP to GMP, were it not for the existence of another, as 

 yet unidentified enzyme, which catalyses the more direct formation of 

 AMP to IMP. The function of this control mechanism could be demons- 

 trated by experiments with [2-i*C]adenine. This compound was con- 

 verted to guanine with considerably lower i^C-content in a histidine-free 

 medium, indicating considerable conversion via AICAR ; in a histidine- 

 containing medium, the guanine had the same radioactivity as the 

 adenine, indicating the more direct conversion via IMP without loss 

 ofCg. 



It is hard to visualize how such a complex network of reactions could 

 be controlled in a simpler fashion. 



Rocker: Can you elaborate on the point of differentiation between 

 inhibition of enzyme activity and enzyme synthesis in reference to your 

 own very interesting studies? Is the case of guanine, which inhibits 

 enzyme synthesis as well as enzyme activity, an exception or a frequent 

 occurrence? 



Magasanik: In general, only the first enzyme of a particular bio- 

 synthetic sequence seems to be subject to inhibition by the ultimate 

 product, while many of the enzymes of the sequence may be subjected 

 to repression. In our laboratory, Mr. A. P. Levin has found that 

 in Salmonella typhimurium guanine represses the formation of IMP- 

 dehydrogenase (the enzyme responsible for the conversion of IMP to 



