STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 



149 



Yates and Pardee (1956) have particularly explored this phenomenon for 

 the control of pyrimidine biosynthesis among pyrimidine-requiring mutants 

 of E. coli. 



Thus, m the sequence studied by these authors, as shown in formula (XI) 



ATP 

 + CO2 

 + NH, 



Carbamyl phosphate 

 Aspartic acid 



Cytosine 

 derivatives 



Nucleic acid 



O' N 



XOOH 



Uracil 



Orotic acid 



(XI) 



mutants were obtained which required uracil or cytosine for growth. In the 

 presence of 10-^ M concentrations of these pyrimidines, the production of 

 ureidosuccinate was inhibited, as well as the subsequent products, dihydro- 

 orotic acid and orotic acid. In one mutant the presence of orotic acid inhibited 

 formation of dihydroorotic acid. Cytosine nucleosides and nucleotides were 

 found to inhibit the condensation of carbamyl phosphate and aspartic acid 

 to form ureidosuccinic acid. In ultraviolet irradiated cells, in which the 

 cytosine compounds were not removed into nucleic acid, the formation of 

 ureidosuccinate was inhibited. In these systems there was apparently 

 inhibition of both enzyme and of its formation. 



Similar phenomena are known in the area of purine and amino acid bio- 

 synthesis. The sequence of arginine biosynthesis in E. coli may be represented 

 as follows: 



ornithine 

 acetylomithinase transcarbamylase 

 glutamate > N-a.- acetylomithine > ornithine > citrulline > 



argmine. 



The presence of exogenous arginine represses the formation of acetylomith- 

 inase in a mutant blocked in the formation of N a-acetylornithine, although 

 the presence of the latter compound induced the formation of the enzyme 

 (Vogel, 1957). 



