62 KOZLOFF ET AL. 



phage protein is built up from simple substrates in rapid equilibrium 

 with the ammonia of the medium. Presumably, these are found in the 

 acid soluble N fraction which is adequate to account for the observed 

 incorporation of bacterial N into phage protein N. 



4. Considering now the precursors of phage DNA: a) P, N, and C 

 of the host all appear in substantial amounts in the virus DNA, and 

 in synthetic medium host N always appears both in greater proportions 

 and greater amounts in phage DNA than in phage protein, b) Bacterial 

 RNA does not turn over after virus infection (Cohen, 1948), c) Host 

 purines are transferred intact to phage DNA (intact since the C^* is 

 found only in the purine carbon of the host and the virus), d) Phage 

 thymine N originates in the host to an equal extent with phage purine 

 N though there is a much larger reservoir of purines ( RNA + DNA) 

 in the bacteria than of thymine (DNA only), and e) There is sufficient 

 N, P, purines, and thymine in the bacterial DNA of the average cell to 

 account for all of the bacterial contribution of these substances to virus 

 DNA, although there is insufficient purine in the initially acid soluble 

 fraction to account for the total of this transfer. Hence, complex sub- 

 strates are utilized for phage DNA synthesis. These may be simple 

 purines but more probably are nucleosides, nucleotides, or polynucleo- 

 tide fragments. However, intact bacterial DNA is probably not trans- 

 ferred to phage DNA (excluded by the double labeling experiment, 

 the different ratios of base transfer, and the tentative conclusion that 

 the two DNA's have different composition) . Sources of these substrates 

 may be the initial acid soluble pool of nucleosides, etc., RNA, and DNA. 

 At present, the insufficiency of the acid soluble pool and the inertness 

 of the RNA seem to eliminate these fractions as major precursors, but 

 the sufficiency of the DNA, the utilization of thymine in equal amounts 

 with purines, and the lack of effect of acid soluble bacterial P turnover 

 on P utilization for virus synthesis all appear to implicate bacterial 

 DNA as the precursor of a substantial fraction or fragment of virus 

 DNA. Presumably, this bacterial DNA is first degraded and then re- 

 built into phage DNA. 



The finding that the host is the source of some of the N, P, and C 

 of the phage does not minimize the significance of the net synthesis of 

 desoxyribonucleoprotein in infected bacteria but rather emphasizes the 

 active participation of the host. It is clear that the simple substrates 

 of the mediimi must be absorbed by the infected bacterium, elaborated 

 into more complex units (amino acids, purines, etc.) presumably by 

 the synthetic mechanisms of the host, the latter are then combined 

 into nucleoprotein molded into specific configurations possibly by in- 

 tervention of genetic governors of the phage. Since previous genetic 



