142 H. GOBIND KHORANA 



Recent studies appearing from different laboratories 96101 have thrown light 

 on the possible enzymic events occurring in the cell upon phage infection 

 that ensure the synthesis of phage DNA. The results can only be briefly 

 summarized. (1) The first report came from Flaks and Cohen 100101 who 

 showed that within several minutes after infection by T2, T4, or T6 phages 

 a new enzyme appears which hydroxymethylates deoxycytidine-5 '-phos- 

 phate to 5-hydroxymethyldeoxycytidine-5 '-phosphate. A mechanism is 

 thus present for the formation of 5-hydroxymethylcytosine, a characteristic 

 component of DNA of T-even phages. (2) Since the polymerase requires 

 all the precursors at the triphosphate level of phosphorylation, the second 

 discovery vital to the synthesis of T2 phage DNA was that some minutes 

 after phage infection, an enzyme which phosphorylates 5-hydroxymethyl 

 cytosine-deoxyribonucleotide to the corresponding triphosphate is in- 

 duced. 96 ' 98, 98a In addition, the levels of kinases for several of the other deoxy- 

 ribonucleoside-5 '-phosphates were also increased except for the corre- 

 sponding enzyme for deoxycytidine-5'-phosphate. 96 (3) A further event oc- 

 curring upon infection is the induction of an enzyme which destroys deoxy- 

 cytidine-5'-triphosphate 96 • " (product of deoxycytidine-5'-phosphate kinase 

 which still can be demonstrated in the infected cell). 96 The products of the 

 action of the new deoxycytidine-triphosphatase are inorganic pyrophos- 

 phate and deoxycytidine-5'-phosphate, the latter being returned, as it were, 

 for the hydroxymethylating enzyme mentioned above under (1). It thus 

 appears that the selection of the base hydroxy met hylcytosine for T2 phage 

 DNA synthesis is ensured by effective removal of the essential precursor for 

 the incorporation of the normal base, cytosine. (4) The DNA's of T-even 

 phages contain glucose linked to the hydroxymethyl groups of 5-hydroxy- 

 methylcytosine. 102 " 104 An enzyme which transfers glucosyl moieties to such 

 groups, presumably at specific sites on the synthesized DNA has been 

 demonstrated in T2 infected cells. 96 The glucosyl donor is the important 

 uridine diphosphate glucose and the transferase is clearly another enzyme 



96 A. Romberg, S. B. Zimmerman, S. R. Romberg, and J. Josse, Proc. Natl. Acad. 

 Sci. U. S. 45, 772 (1959). 



97 J. F. Roerner and M. S. Smith, Federation Proc. 18, 264 (1959). 



98 R. Somerville and G. R. Greenberg, Federation Proc. 18, 327 (1959). 



98a R. Somerville, R. Ebisuzaki, and G. R. Greenberg, Proc. Natl. Acad. Sci. U.S. 

 45, 1240 (1959). 



99 J. F. Roerner, M. S. Smith, and J. M. Buchanan, J.Am. Chem. Soc. 81, 2594 (1959). 



100 J. G. Flaks and S. S. Cohen, Biochim. et Biophys. Acta 25, 667 (1957). 



101 J. G. Flaks and S. S. Cohen, Federation Proc. 17, 220 (1958). 



102 M. A. Jesaitis and W. F. Goebel, Cold Spring Harbor Symposia Quant. Biol. 18, 

 205 (1953). 



103 R. L. Sinsheimer, Science 120, 551 (1954). 

 i°< E. Volkin, /. Am. Chem. Soc. 76, 5892 (1954). 



