INTRACELLULAR MULTIPLICATION OF BACTERIAL VIRUSES 257 



maturation keep pace with one another. The kinetic pool size estimate is 

 thus in excellent agreement with the value of 40-80 phage equivalents of 

 HMC-containing DNA obtained by direct chemical analysis, which we had 

 already noted as being present within the infected cells. Hershey could show, 

 furthermore, that maturation is efficient, in that about 90 % of the labeled 

 phosphorus introduced early into the pool is later incorporated into intact 

 phages, and that maturation is irreversible, in that phosphorus in mature, 

 intracellular phages does not undergo exchange with the phosphorus of the 

 DNA jjrecursor pool. 



2. Origin of Progeny Protein 



The synthesis of the phage protein was studied by means of yet another 

 modification of Cohen's tracer experiment. For this purpose, W^ or C^*- 

 lysine-labeled bacteria were infected with T-even phages and phage develop- 

 ment allowed to proceed in nonlabeled medium before the protein of the 

 phage progeny was analyzed for its specific content of the labeled isotope. 

 It was found that very little, if any, of the viral protein is derived from the 

 protein of the host cell, i.e., that the bacterial contribution to the protein is 

 much less than to the DNA of the bacteriophage (Kozloff e^ aL, 1951; Siddiqi 

 et at., 1952). The kinetics of assimilation from growth medium of the raw 

 materials for the phage protein were studied by Hershey et al. (1954), who 

 exposed infected bacteria to S^^ label during 5-minute intervals at various 

 stages after the onset of phage development and then determined the amount 

 of S^^ subsequently incorporated into virus and into the general intrabacterial 

 protein. This study showed that the total amount of label which enters 

 intrabacterial protein during the 5-minute exposure to labeled growth 

 medium is more or less the same at all stages of the latent period, as well as 

 also being equal to the labeling attained during the same interval by an 

 uninfected bacterial culture, thus confirming an earlier finding by Cohen 

 (1948a) that total protein synthesis proceeds in the host cell at the same rate 

 before as after infection. The proportion of the total S^^ assimilated which 

 finds incorporation into virus protein, however, varies greatly at different 

 stages of the latent period. Very little of the sulfur assimilated during the 

 first 5 minutes after infection ultimately enters the protein of the phage 

 progeny, whereas 50-60 % of the total sulfur assimilated during any 5-minute 

 interval after the tenth minute of phage development is directed toward 

 mature virus particles. These results indicate that two classes of protein, one 

 a phage precursor and the other not, are synthesized in the infected host cell 

 and that the maximal rate of percursor protein sjoithesis is not reached mitil 

 the latter part of the eclipse period. It can be inferred from considerations 

 similar to these already described for the interpretation of the P^^ assimilation 

 data, that sulfur atoms spend an average development time of about 2 minutes 

 VOL. II — 17 



