56 KOZLOFF ET AL. 



undergo one or two divisions (for N^^) so that the P or N fractions were 

 now equally labeled), c) specifically labeled with C^* in the purine 

 carbon (by growth in medium containing isotopic adenine), or d) 

 doubly labeled with P^^ and N^^ (by growth in medium containing 

 both isotopes). 



Origin of Virus Phosphorus. — In agreement with Cohen (1948), 

 already published work (Kozloff and Putnam, 1950) on the origin of 

 virus P has shown: 1) There is after infection a net synthesis of des- 

 oxyribonucleic acid (DNA) ; and 2) The inorganic phosphate of the 

 medium is the ultimate source of 70-80% of the P of T2, T4, and T6 

 bacteriophages. Further, our work has shown 3) Quantitatively, only a 

 small fraction of the bacterial P is utilized for virus synthesis, and 4) 

 The latter fraction is apparently not acid soluble P but appears to be a 

 stable and possibly specific precursor of phage DNA. From this work 

 the hypothesis was advanced (Kozloff and Putnam, 1950) that much of 

 the bacterial DNA is either transferred intact or in degraded form for 

 synthesis of some of the phage DNA. The experiments with N^^ and 

 C^* are compatible with this hypothesis but do not yet rigorously ex- 

 clude bacterial RNA N, P or purines as a source of some of the 

 phage DNA. 



Origin of Virus Nitrogen. — In 14 experiments, summarized in 

 Table I, it was found that the medium is the source of the major part 

 of virus N, but that also bacterial N appears in both phage nucleic 

 acid and protein. Quantitatively, only a small fraction of the bacterial N 

 ends up in the phage. More bacterial N was found in the virus nucleic 

 acid than in the virus protein. Thus, for whole phage, from 11.6% to 

 38.8% of the N was derived from the host with a mean value of about 

 20%; for the virus nucleic acid, from 16-43% of the N came from the 

 host, and for phage protein, from 6 to 27% of the N was bacterial N. 

 In all cases but one (Expt. 14) (performed in broth) more virus 

 nucleic acid N came from the host than did virus protein N. 



The above summary emphasizes the variability of the proportion 

 of host N in the virus but does not show the effects of 5deld of virus, 

 composition of the medium, or time of liberation on the isotope distribu- 

 tion in the virus. All these effects were investigated, and in Table I 

 the experiments are listed in order of increasing average yield of virus 

 per infected bacterial cell. The data show an inverse relationship of 

 the per cent of phage protein N derived from the host with yield of 

 virus but no correlation of the latter factor with the per cent of phage 

 nucleic acid N derived from the host. These results were first estab- 



