INTRACELLULAR MULTIPLICATION OF BACTERIAL VIRUSES 255 



served also as a model for resolving other precursor questions e.g., that which 

 had posed itself to the students' of induced enzyme synthesis (Hogness et al., 

 1955). Cohen grew two bacterial cultures, one in a medium labeled with a 

 given specific activity of P^^ and the other in a nonlabeled medium. After 

 growiih, the bacteria were ccntrifuged out of their original culture fluids, the 

 labeled cells resuspended in nonlabeled growth medium, and the nonlabeled 

 cells resuspended in labeled growth medium. Both cultures were then 

 multiply infected with T-even phages, incubated until complete lysis, and 

 the phage yields isolated, purified, and assayed for their specific P^- contents. 

 The result of Cohen's experiment was that the DNA-phosphorus of the phages 

 grown on labeled bacteria in nonlabeled medium possessed only 1/3 of the 

 specific activity of the P^^ introduced, whereas the DNA-phosphorus of the 

 phages grown on nonlabeled bacteria m labeled medium possessed 2/3 of the 

 specific activity of the P^^ introduced. Cohen thus demonstrated that most of 

 the phage DNA is synthesized from materials still in the growth medium at 

 the moment of infection, i.e., that the phage particles cannot have been 

 derived from pre-existing bacterial precursors. Cohen's experiment was later 

 repeated, using the nitrogen isotope N^^ as the label, by which means it could 

 be shown that most of the nitrogen of the phage DNA, hke its phosphorous, 

 is also derived from materials stiU in the growth medium at the moment of 

 infection (Kozloff et at., 1951). Contrary to what had been supposed at first 

 (Cohen, 1948b), the fraction of the atoms of the phage DNA assimilated 

 before infection, or the bacterial contribution, is not preferentially derived 

 from low molecular weight intermediates of bacterial nucleic acid metaboHsm 

 present in the host cell at the moment of infection (Kozloff and Putnam, 1950; 

 Stent and Maaloe, 1953). Instead, it could be shown by infecting bacteria 

 labeled with radiocarbon C^^ only in their pyrimidines (Weed and Cohen, 

 1951), or exclusively in their thymine (Kozloff, 1953; Hershey et al., 1954) 

 that the bacterial nucleic acids, or, more specifically, the bacterial DNA, is 

 the chief provenance of the bacterial contribution, (A minor fraction of the 

 bacterial contribution is also derived from the host ribonucleic acid (Hershey 

 et al., 1954).) The utilization of host nucleic acid for the bacterial contribution 

 involves breakdown of the bacterial DNA into low molecular weight sub- 

 stances subsequently repolymerized into phage-specific polynucleotides 

 (Hershey et al., 1953, 1954). Since the composition of the bacterial DNA is 

 very different from that of the T-even phages, in particular in that the former 

 contains cytosine and the latter HMC, any biochemical route for the bacterial 

 contribution other than breakdown and resynthesis would, in any case, be 

 difficult to imagine. In the course of the latent period more than 85 % of the 

 host (i.e., cytosine-containing) DNA disappears, most of which ultimately 

 reappears in phage (i.e., HMC-containing) DNA (Hershey, 1953a; Hershey et 

 al., 1954), host cytosine having been converted into viral HMC (Cohen, 1953). 



