214 ROBERT L. SINSHEIMER 



It is also pertinent to point out that during T2 phage infection thymidylic 

 acid can be synthesized in normally thymine-requiring cells. 123 Cohen has 

 shown that the activity of the enzyme system, which in the presence of 

 formaldehyde and tetrahydrofolic acid converts dUMP to TMP (thymidylic 

 synthetase), is increased seven- to eightfold in T2-infected, normal coli 

 cells. In thymine-requiring cells, this enzyme system is not detectable until 

 after phage infection. 124 



Keck, Mahler, and Fraser 125 have described a dCMP deaminase which 

 begins to appear 3 to 5 minutes after infection with T2 phage. By convert- 

 ing dCMP to dUMP the enzyme provides a precursor of thymidylic acid, 

 and may be involved in the phage-directed synthesis. Thus, dCMP may be 

 converted to either dHMP or to TMP via dUMP. 



c. RNA Metabolism During Phage Infection 



Net RNA synthesis ceases very shortly after the initiation of phage 

 infection. "*• 126 - 128 However, P 32 experiments have indicated that there is 

 a significant turnover of RNA after infection. 6 ' 129 - 132 The amount, rate, 

 and duration of this turnover appear to be strongly dependent upon the 

 composition of the external medium. Upon centrifugal fractionation of 

 the cells, the phosphate assimilated into RNA was shown to be present in 

 both of two particulate fractions and in a "soluble" fraction, although 

 with varying specific activity. 130 



Pulse P 32 experiments indicate that this P 32 can pass from RNA into 

 phage DNA. This process is accelerated if the cells are washed to deplete 

 their acid-soluble pools. However in T2 infection the rate of turnover of 

 the RNA is not sufficient at any time to account for the rate of production 

 of phage DNA; this is particularly true in the later stages of infection. 

 Thus if RNA is a precursor of the phage DNA, it could not be the only pre- 

 cursor. However, the experiments do demonstrate that the transfer of ma- 

 terial from RNA to DNA does not involve degradation to any level lower 

 than that of nucleotides. 



In contrast, in T7 infection, the incorporation and turnover of P 32 into 



23 H. D. Barner and S. S. Cohen, J. Bacteriol. 68, 80 (1954). 



24 S. S. Cohen, Abstr. 134th Meeting Am. Chem. Soc. p. 22c (1958). 



2B K. Keck, H. R. Mahler, and D. Fraser, Arch. Biochem. Biophys., 86, 85 (1960). 



26 S. S. Cohen, J. Biol. Chem. 174, 2S1 (1948). 



27 L. M. Kozloff, K. Knowlton, F. W. Putnam, and E. A. Evans, Jr., J. Biol. Chem. 

 188, 101 (1951). 



28 L. A. Manson, J. Bacteriol. 68, 703 (1953). 



29 E. Volkin and L. Astrachan, Virology 2, 149 (1956). 



29a I. Watanabe, Y. Kiho, and K. Muira, Nature 181, 1127 (1958). 



30 E. Volkin and L. Astrachan, Virology 2, 433 (1956). 



31 E. Volkin, L. Astrachan, and J. L. Countryman, Virology 6, 545 (195S). 

 L. Astrachan and E. Volkin, Biochim. el Biophys. Acta 29, 536 (1958). 



