338 GEORGE W. CROSBIE 



intact units into DXA-thymidine residues has likewise been repeatedly ob- 

 served. 99-103, 118 



The 5-methyl group of mammalian DNA would appear to have its origin 

 in a "1-C" unit derivable from formate-C 14 , 119 f ormaldehyde-C 14 , 120 ■ 121 

 serine-3-C 14 , 122 glycine-2-C 14 , 119 or methionine-methyl-C 14 . 123 - 124 Elwyn and 

 Sprinson 122 have shown, using L-serine-2,3-D,3-C 14 ,N 15 , that the pathway 

 of incorporation of the (8-carbon does not involve a derivative at the oxida- 

 tion level of formate. Lowy et al. 1 ' 20 have shown that C 14 , D-labeled formal- 

 dehyde (a mixture of C 14 H 2 0, CD 2 0, and CHDO) is incorporated into the 

 rat DNA methyl group with a considerable loss of D relative to C 14 . The au- 

 thors have pointed out, however, that the obvious significance of the results 

 may be vitiated by isotope selection effects (cf. Rachele et al. nb and Abe- 

 les 126 ). Crosbie 127 has shown that formate-C 14 is utilized for the "1-C" posi- 

 tions of the purine ring (positions 2 and 8) but not for the thymine methyl 

 group of E. coli DNA. 



Green and Cohen 117 and Crosbie 127 have shown using methionine-less mu- 

 tants of E. coli that methionine is not a methyl donor in thymine ring bio- 

 synthesis. This conclusion is confirmed by the nonutilization of methionine- 

 methyl-C 14 for DNA-thymine synthesis in a uracil-less mutant of E. coli. 117 

 S-Hydroxymethylhomocysteine does not appear to be a precursor of the 

 methyl group of thymine (or of the 5-hydroxy methyl group of viral 5-hy- 

 droxymethylcytosine). 117 It would appear 124 that in systems in which methi- 

 onine-methyl utilization has been reported the pathway of incorporation 

 involves a prior conversion of the — S — Me group to the active donor, 

 probably a derivative of folic acid (vide infra). 



A kinetic study of glycine-2-C 14 and serine-3-C 14 incorporation into ex- 

 ponentially growing cells of E. coli has revealed 127 that glycine and serine 

 do not lie on the pathway of incorporation of serine-3-C 14 and glycine-2- 

 C 14 , respectively, into the thymine methyl group. 



The evidence of incorporation experiments points to a role of Af-hydroxy- 

 methyltetrahydrofolic acid (N— CH 2 OH— FH 4 )— or the A^ 5 ,AT 10 -methyl- 



118 H. Amos and B. Magasanik, J. Biol. Chem. 229, 653 (1957). 



119 J. R. Totter, E. Volkin, and C. E. Carter, J. Am. Chem. Soc. 73, 1521 (1951). 



120 B. A. Lowy, G. B. Brown, and J. R. Rachele, J. Biol. Chem. 220, 325 (1956). 



121 R. L. Hamill, R. L. Hermann, R. U. Byerrum, and J. L. Fairley, Biochim et Bio- 

 phys. Acta 21, 394 (1956). 



122 D. Elwyn and D. B. Sprinson, J. Biol. Chem. 207, 467 (1954). 



123 R. L. Herrmann, J. L. Fairley, and R. U. Byerrum, J. Am. Chem. Soc. 77, 1902 

 (1955). 



124 S. Kit, C. Beck, O. L. Graham, and A. Gross, J. Biol. Chem. 233, 944 (1958). 



125 J. R. Rachele, E. J. Kuchinskas, F. H. Kratzer, and V. du Vigneaud, J. Biol. 

 Chem. 215, 593 (1955). 



126 R. H. Abeles, Federation Proc. 14, 170 (1955). 



127 G. W. Crosbie, Biochem. ./. 69, IP (1958). 



