STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 



181 



Mechanisms for the production of di- and triphosphates of the ribonucleo- 

 tides have been described earher. The mechanism of the formation of the 

 purine deoxyribotides is as yet quite obscure. Degradative mechanisms for 

 purines are described m aU textbooks of biochemistry. 



c. Biosynthesis of Pyrimidine Nucleotides. The formation of orotic acid and 

 uracil via the condensation of carbamyl phosphate and aspartic acid to 

 ureidosuccinate and dihydroorotic acid has been presented earher, in the dis- 

 cussion of feedback mechanisms in the control of enzyme production. The 

 condensation of orotic acid and uracil with l-pyrophosphoryl ribose-5-phos- 

 phate to form orotidylic acid and uridylic acid, respectively, has been de- 

 scribed (Romberg, 1957a,b). These are separate reactions whose existence 

 and use are determined in particular organisms by the presence or absence of 

 an orotic acid decarboxylase or an orotidyhc acid decarboxylase. Thus, 

 organisms are known in which orotic acid is converted first to orotidyhc 

 acid and decarboxylated to uridylic acid. In others, orotic acid is first de- 

 carboxylated to uracil prior to condensation to uridyhc acid. In still other 

 organisms, both pathways exist side by side. These relations are presented in 

 formula (XXIX). 



carbamyl phosphate ^ 



+ 1- 



aspartate J 



-> ureidosuccinate 

 lu-acil 



dihydroorotic acid 

 orotic acid 



-CO3 



Cytidylic acid 



UTP -^ Uridylic acid 

 (XXIX) 



N' , L L ,° N' XOOH 



I 1-pyrophosphoryl I 



i nbose-S-P \ 



~'^Q^ Orotidylic acid 



As presented in the schema, the formation of the cytosine ribonucleotide 

 series occurs at the triphosphate level in the amuiation of UTP to CTP 

 (Lieberman, 1956). This phenomenon virtually isolates cytosine metabolism 

 at the end of a reaction chain which must be approached through uracil de- 

 rivatives. At the present time, no mutants are known to have cytosine re- 

 quirements without concomitant uracil requirements. Cytosme wiU not 

 react with ribose-1 -phosphate to give a nucleoside or with 1-pyrophosphoryl 

 ribose-5-phosphate to give a cytosme nucleotide. Thus, the pyrimidine ring 

 of cytosine must be deaminated to uracil before scavenging is possible. 

 However, some sUght evidence exists to indicate that cytosine nucleosides 

 may be phosphorylated to the nucleotide. 



