340 GEORGE W. CROSBIE 



revealed that deoxyuridine like thymidine is utilized almost exclusively for 

 DNA-thymidine formation. No incorporation of deoxyuridine into DNA- 

 cytosine was noted. Uridine was utilized extensively for polynucleotide 

 uracil, cytosine, and thymine synthesis whereas 5-methyluridine, like thy- 

 mine, showed a small but significant incorporation into DNA-thymine of 

 regenerating liver (but not of intestinal mucosa). The evidence suggests that 

 deoxyuridine (or a nucleotide derivative) is the primary "1-C" acceptor 

 molecule in thymine ring biosynthesis. Friedkin and Roberts 97 have de- 

 scribed a similar aminopterin-inhibited incorporation of deoxyuridine into 

 the DNA-thymine of suspensions of chick embryo and bone marrow cells. 

 Prusoff and co-workers 43, 139 have shown that deoxyuridine, deoxycytidine, 

 and the corresponding ribonucleosides increase the incorporation of formate- 

 C 14 into DNA-thymine of rabbit bone marrow and Ehrlich ascites carcinoma 

 cells. The cytosine nucleosides are markedly more efficient than the corre- 

 sponding uracil derivatives. This evidence together with the observation 114 

 of 5-methyldeoxycytidine deaminase activity in E. coli extracts has been 

 interpreted as indicating the possible role of deoxycytidine (or a nucleotide 

 derivative) as a "1-C" acceptor molecule in thymine-ring formation. 



The isotopic evidence on which this conclusion is based cannot, however, 

 be considered as unequivocal owing to the difficulties inherent in the inter- 

 pretation of the results of incorporation studies in whole-cell systems in 

 which permeability, pool size, and kinetic factors of unknown character 

 and magnitude may operate. It may be noted that Kit et al. ni have shown 

 that acid-soluble 5-methyldeoxycytidine and the corresponding 5'-deoxy- 

 nucleotide are not labeled during the incorporation of formaldehyde-C 14 

 into the DNA-thymine of lymphatic-cell suspensions supplemented with 

 5-methyldeoxycytidine. 



The intact incorporation of cytidine-G-C 14 and uridine-G-C 14 into the de- 

 oxy nucleotide residues of mammalian, avian, and bacterial DNA clearly 

 indicates as previously discussed that the conversion of a ribonucleoside 

 (or ribonucleotide) to the corresponding deoxyribonucleoside (or deoxy- 

 ribonucleotide) can occur without fission of the A^-glycoside linkage and 

 without effective rupture of the pentose chain. 99-103 These conclusions are 

 amplified by the studies of the distribution of activity in the polynucleotide 

 ribose and deoxyribose chains of E. coli grown on a variety of C 14 -substrates 

 as carbon source. Thus Bernstein and Sweet 140 using lactate- 1-C 14 have 

 found the pattern of activity in C-l to C-5 of deoxyribose and ribose to be 

 21, 12, 62, 4, 3 and 10, 15, 69, 1, 1, respectively. These data, more so than 

 those obtained in experiments with specifically labeled glucose, show that 



139 W. H. Prusoff, L. G. Lajtha, and A. D. Welch, Biochim. et Biophys. Ada 20, 209 

 (1956). 



140 1. A. Bernstein and D. Sweet, Federation Proc. 17, 190 (1958). 



