Control of Cell Metabolism in Bacteria 297 



these four effects is well understood in principle: the increase 

 in rate of enzyme activity as substrate concentration is 

 increased. Implications for metabolism have been discussed 

 at length (Dixon, 1949) and will not be commented upon 

 further here. The next sections will deal with experimental 

 examples of the other three cases: inhibition of enzyme 

 activity, induction and repression of enzyme synthesis. 



Enzyme inhibition 



Inhibition of a reaction by one of its products would be 

 most advantageous for preventing surplus production of 



Nutrients 



Fig. 1. Schematic representation of a metabolic pathway. 



Po is a permease ; A, F are metabolites ; Eo, E^, Ep 



are enzymes; and X^, Xb are the "systems" for synthesis of 

 Ea, Eb, etc. 



metabolites. Especially useful would be an inhibition of an 

 early reaction of a metabolic sequence by a product near the 

 end of the sequence: such a "feedback inhibition" would 

 serve to prevent production of any intermediate in the entire 

 sequence when excess product is present. In Fig. 1, the 

 inhibition of the conversion of A to B by D would represent 

 such a case. 



Several examples of feedback inhibition have been found in 

 bacteria. A case which has been extensively studied is that of 

 the inhibition of the pathway of pyrimidine synthesis (Yates 

 and Pardee, 1956). The existence of this feedback control, 

 active during metabolism, was discovered in the course of 

 studies on a mutant of Esch. coli that required a pyrimidine 

 such as uracil for growth. This mutant possessed the enzyme 



