PRECURSORS OF PHAGE 6l 



or guanine had to be added to a minute amount of the phage purine 

 with the result that the specific activity of each of the purines could 

 not be determined. In this experiment, with low phage yield in mini- 

 mal medium, about 50% of the phage purines were derived from host 

 material assimilated prior to infection. In the second experiment a 

 minute amount of chromatographically pure labeled adenine was in- 

 corporated in the host cells. The washed cells were infected in N-rich 

 medium (0.1% NH4CI). The adenine and guanine of the resulting 

 virus were isolated chromatographically (presumably recovered quan- 

 titatively), the amounts determined spectrophotometrically, and the 

 activity measured after the addition of carrier. The data of Table V 

 indicate that 14% of the virus adenine and 20% of the virus guanine 

 arose from material in the host prior to infection. It is noteworthy that 

 the total radioactivity of the phage adenine (wt. in mg. x thick sample 

 count) is almost equivalent to the total radioactivity of the phage 

 guanine. The preliminary data on recoveries indicate that there is a 

 net synthesis of DNA purines (total purine of liberated phage equals 

 twice the DNA purines of the host cells) and also suggest a remarkable 

 difference in the chemical composition of phage DNA and bacterial 

 DNA. 



DISCUSSION AND CONCLUSIONS 



1. Direct isotopic investigation confirms that constituents still in 

 the medivim at the time of infection are the ultimate source of most of 

 the N, P and C of bacteriophage. This is incompatible with the pre- 

 cursor theory of virus reproduction in its crudest form. 



2. Quantitatively, only a small fraction of the bacterial P (about 

 one-twelfth) and of the bacterial N (about one-twentieth) is used up 

 for phage synthesis. 



3. Considering now the precursors of protein N, a) Phage protein 

 synthesis is reported to precede phage DNA synthesis (Cohen, 1948), 

 but phage DNA from labeled cells contains more isotope than does 

 phage protein, b) Bacterial N incorporation into phage diminishes with 

 time of sampling following an exhaustible substrate type of curve de- 

 spite the ample supply of isotope in the host protein, c) Bacterial N ap- 

 pears about equally in all amino acid fractions, and d) Enzyme adapta- 

 tion in E. coli ceases on phage infection (Monod and Wollman, 1947); 

 infected bacteria no longer divide or grow, they produce only nucleo- 

 protein equivalent to phage (Cohen, 1948), and e) The isotope content 

 of the debris is 70% that of the infected bacterium. The facts c, d, and 

 e suggest that protein turnover ceases in infected bacteria and pro- 

 teolysis follows rather than precedes elaboration of the phage. Hence, 



