3G. BIOSYNTHESIS OF PYRIMIDINE NUCLEOTIDES 



337 



5 10 , HCHO 

 N,N -methylene- FH 4 -« 



deoxycy tid ine -5- 

 phosphate 



CHoOH 



5 - hydroxymethyl - 



deoxycy tidine - 5 - 



phosphate 



Fig. 3. The 5-hydroxymethyldeoxycytidine-5'-phosphate synthetase reaction 

 (R = 5'-phosphodeoxyribosyl). 



cells of E. coli. A requirement for tetrahydrofolic acid (FH 4 ) was demon- 

 strated using Dowex-l-(chloride)-treated enzyme. The reaction may be 

 envisaged as a Mannich-type base formation followed by hydrolysis (Fig. 

 3) or as a Knoevenagel-type reaction. 



The possible biosynthetic relationship between 5-hydroxymethyl and 

 5-methyl pyrimidine derivatives will be considered later. Little is known of 

 the mechanism of biosynthesis of the 5-methylcytosine ring. Normally con- 

 sidered as a constituent of plant and mammalian DNA's, 5-methylcytosine 

 has recently been isolated as the 3'(2'-)-ribonucleotide from the RNA of 

 E. coli K 12. 113 



It may be noted that 5-methyldeoxycytidine is a substrate of a purified 

 deoxycytidine deaminase of E. coli ni and by virtue thereof is capable of sup- 

 porting the growth of the thymine-less mutant, E. coli 15T-, at a rate equal 

 to that supported by thymidine. 



IV. Biosynthesis of Thymidine Nucleotides 



1. Biosynthesis of Thymidine-5'-phosphates 



a. Biosynthesis in Whole Cell Systems 



Considerable evidence exists pointing to the role of uracil or cytosine 

 derivatives as precursors of polynucleotide thymine. Orotic acid 115 is a 

 known precursor and uracil is utilized for DNA-thymine synthesis in uracil- 

 less mutants of E. coli. n6t m The incorporation of cytidine and uridine as 



113 H. Amos and M. Korn, Biochim. et Biophys. Acta. 29, 445 (1958). 



114 S. S. Cohen and H. D. Barner, J. Biol. Chem. 226, 631 (1957). 



115 P. Reichard, Acta Chem. Scand. 3, 422 (1949). 



116 A. M. Moore and J. B. Boylen, Arch. Biochem. Biophys. 54, 312 (1955). 



117 M. Green and S. S. Cohen, J. Biol. Chem. 225, 387 (1957). 



