I. REPLICATION OF DNA IN CELL-FREE SYSTEMS 



TABLE II 



Formation of Deoxyuridine Phosphates from 

 Different Uridine Phosphates"'' 



Deoxyuridine phosphates formed 

 Substrate (niMmoIes) 



UMP (28 mMmoles) . 003 



UDP (36 mMmoles) 0.340 



UTP (38 niMmoles) 0.024 



" From Bertani et al. (1961). 



'' The reaction mixture contained the different C'^-uridine nucleotides (11 X 10" cpm 

 per Mniole) together with 0.5 ^niole of ATP, 1.2 ^umoles of MgClo, 0.34 /xmole of reduced 

 lipoic acid, 2.3 Aimole of tris-HCl buffer, i)H 7.85, 0.1 yumole of mercaptoethanol, and 

 0.16 mg of fraction B, in a final volume of 0.1 ml. 



(1952) should not be overlooked. This latter enzyme catalyzes the 

 following reaction: 



Pyrimidine deoxyriboside + purine ^ purinedeox.\riboside + pyrimidine 



The importance of this reaction in the de novo synthesis of purine deoxy- 

 ribonucleotides remains to be evaluated. 



A pathway for the formation of deoxyribose, not involving nucleoside 

 derivatives, was described by Racker (1952), who demonstrated the 

 condensation of acetaldehyde and glyceraldehyde-3-phosphate to form 

 deoxyribose-5-iihosphate. This reaction, catalyzed by enzymes from 

 E. coli or animal tissues (deoxyriboaldolase) could be coupled to the 

 phosphodeoxyribomutase of Manson and Lampen (1951) and the nucleo- 

 side phosphorylase of Friedkin and Kalckar (1950) and Friedkin and 

 Roberts (1954) to form deoxyribonucleosides according to the following 

 scheme: 



aldolase 



Acetaldehj'de + glyceraldeh3de-3-phosphate ;:^ deoxyribose-5-phosphate 



mutase 



Deoxyribose-5-phosphate ;=^ deoxyribose-1-phosphate 



pho.iphorylase 

 Deoxyribose- 1 -phosphate -|- base ^ deox^Tibomicleoside -|- phosphate 



The equilibrium of the aldolase reaction is very unfavorable toward 

 synthesis, suggesting that the significance of this reaction may lie in the 

 degradation of deoxyribose-5-phosphate. 



In analogy to ribonucleotide synthesis in which 5-phosphoribosyl-l- 

 pyrophosphate is the active pentose intermediate (Kornberg et al., 1955; 

 Remy et al., 1955), one could reason that a deoxyribose analog of this 

 compound might participate in deoxy ribonucleotide synthesis: 



