THE ROLE OF ENZYME REGULATION IN METABOLISM 



81 



Furthermore, not one enzyme alone but several enzymes of the path- 

 way were repressed. Thus, arginine not only repressed acetylornith- 

 inase formation but also the next two enzymes of the pathway— 

 ornithine transcarbamylase (Gorini and Maas, 1957) and argininosuc- 

 cinase ( Gorini and Maas, 1958 ) . The level of the transcarbamylase was 

 measured after growth of arginine-requiring mutants on limiting 

 amounts of arginine; the supply of arginine rather than the growth 

 rate was the important factor. 



In another extensively studied case, foiTnation of the first three en- 

 zymes of the pyrimidine biosynthetic pathway of E. coli was repressed 

 by an excess of added or endogenously formed pyrimidines ( Yates and 

 Pardee, 1957)— see Figure 2. Under normal conditions of growth the 

 concentrations of these enzymes are far lower than the maximal abili- 

 ities of the bacteria to synthesize them. About 50 molecules of the first 

 enzyme, aspartate transcarbamylase, are normally found per bacterium, 

 whereas if growth conditions are arranged so that repression is released, 

 the rate of enzyme synthesis relative to total protein synthesis rises at 

 least 2,000-fold. The enzyme finally makes up as much as 7 per cent of 

 the total protein ( Shepherdson and Pardee, 1960 ) . The rate of enzyme 







10 20 30 



MINUTES AFTER REMOVAL OF URACIL 



40 



Figure 2. Kinetics of aspartate transcarbamylase repression and its release. 



