256 G. S. STENT 



The inferred extensive breakdown of the host DNA is in harmony with direct 

 cytological observations on T-even infected bacteria, which reveal that the 

 host nucleus disappears at early stages of intracellular phage development 

 (Luria and Human, 1950; Murray et al., 1950). 



The kinetics of assimilation of the phosphorus of phage DNA have been 

 studied by another extension of Cohen's original tracer experiment. For this 

 purpose, P^^ label was either added to a nonradioactive bacterial culture or 

 withdrawn from a P^^-labeled bacterial culture at various times before or 

 after its infection under one-step growth conditions. The fraction of the total 

 phage phosphorus assimilated by the time of addition or removal of the P^^ 

 label could then be estimated from the specific P^^ activity of the DNA- 

 phosphorus of the phage yield produced (Stent and Maaloe, 1953). The result 

 of this experiment was that, prior to infection, the bacteriophage phosphorus 

 is assimilated at the rate of bacterial growth, indicating once more that end 

 products of bacterial synthesis, like the nucleic acids, and not metabolic 

 intermediates, are the source of the bacterial contribution. Immediately 

 after infection, however, the rate of phage phosphorus assimilation was seen 

 to increase sharply, indicative of a sudden alteration in the metabolism of the 

 infected cell which channels almost all of the newly assimilated phosphorus 

 atoms into the pathway of phage DNA synthesis. Assimilation was foimd to be 

 complete within 20 minutes after infection, although mature progeny phage 

 continue to make their appearance for at least 10 more minutes. A further 

 variable was introduced into this experiment by inducing lysis of the infected 

 cultures from time to time and determining how the phosphorus assimilated 

 at different stages of the growth cycle is distributed over phage progeny 

 maturing at various times. The results obtained indicated that the bacterial 

 contribution appears preferentially, though not exclusively, in the first 

 progeny to mature, that phosphorus atoms assimilated after infection spend 

 an average "development time" of 14 minutes in the cell prior to their 

 incorporation into the DNA of infective phages, and that individual develop- 

 ment times of different phosphorus atoms are widely distributed around this 

 average. This apparent mixing of phosphorus of the bacterial contribution 

 and of phosphorus assimilated at various times after infection led to the idea 

 that phosphorus of the phage DNA passes through one or more intrabacterial 

 precursor pools, whose size and number is reflected by the observed degree of 

 mixing (Stent and Maaloe, 1953). Hershey (1953a) carried out further experi- 

 ments on the kinetics of the transport of phosphorus from the culture medium 

 and the bacterial DNA to the viral progeny. He inferred from his observations 

 that at the end of the eclipse period the DNA of the phage particles about to 

 mature is withdrawn from a pool amounting to 50-100 phage equivalents of 

 phage precursor DNA, which pool then maintains a more or less constant 

 size durmg the remainder of the latent period, while DNA synthesis and 



