298 Arthur B. Pardee 



necessary for synthesis of orotic acid (an intermediate of 

 pyrimidine synthesis), but could not convert orotic acid to 

 pyrimidines. It was observed that orotic acid was not pro- 

 duced by the bacteria while they were growing in the presence 

 of uracil. But as soon as the uracil was used up, orotic acid 

 and the two intermediates of orotic acid synthesis were found 

 in the medium. Clearly, some condition in the growing 

 bacteria blocked the first specific step of the metabolic path- 

 way. Further experiments with whole cells showed that it 

 was the presence of a pyrimidine that inhibited the appearance 

 of these compounds. Finally, it was shown with cell-free 

 preparations of the organism that a product at the end of the 

 sequence (cytidylic acid) was quite a good inhibitor of the 

 first specific step, i.e. the condensation of aspartate and 

 carbamyl phosphate. The inhibition is competitive; and 

 therefore the activity of the enzyme in vivo must depend on a 

 balance between substrates and inhibitor. This inhibition is 

 direct, and not an equilibrium situation in which the reaction 

 is inhibited by a surplus of products acting through all of the 

 intermediate steps, as shown by the in vitro result and also by 

 the inhibition in vivo in mutants which lack intermediate 

 enzymes. 



Feedback inhibitions have been discovered for a number of 

 other pathways including those for isoleucine (Umbarger, 

 1956), purines (Gots, 1957) and threonine (Wormser and 

 Pardee, 1958). Also, numerous observations regarding limited 

 production of metabolic intermediates by mutant bacteria 

 (see Novick and Szilard, 1954), and utilization of nutrients 

 supplied in the medium preferentially to endogenous products 

 (Roberts et ah, 1955) suggest feedback controls. In general, 

 these data do not permit one to distinguish between inhibition 

 of enzyme activity and inhibition of enzyme formation 

 (repression), to be described below. 



Enzyme induction 



It is well known that the concentrations of some enzymes in 

 bacteria are not constant but depend very strikingly upon 



