264 G. S. STENT 



and DNA, by means of their "Blendor experiment." This experiment consists 

 of infecting bacteria with P^^-, S^^-, or C^^-labeled phage particles, separating 

 the infected cells from any unadsorbed phages by centrifugation and resuspen- 

 sion in a salt-poor medium, and agitating the infected suspension violently 

 for a short time in a Waring Blendor. The infected bacteria are then centri- 

 fuged once more in order to examine how much of the adsorbed radioactivity 

 has been stripped off the cells by agitation, i.e., how much remains in the 

 supernatant fluid, and how much has still remained attached to the cells, i.e., 

 is sedimented into the bacterial pellet. The result of this experiment was that 

 more than 80 % of the viral sulfur (mostly in methionine) and amino acid 

 carbon is removed by the Blendor treatment, but that more than 80 % of the 

 viral phosphorus or purine-pyrimidine carbon remains attached to the cells. 

 Viability assays are also made before and after agitation; they show that, m 

 spite of the extensive removal of viral protein, practically all of the infected 

 bacteria can continue to produce mature phage progeny. It appears from 

 these results that most of the viral protein remains at the surface of the 

 infected cell and that most of the viral DNA enters the cell at the outset of the 

 infection. Hershey and Chase (1952) further substantiated the inference that 

 in the process of entering the host cell the phage DNA leaves its protein 

 envelope by showing that adsorption of phages to bacterial cell- wall debris or 

 to heat-killed bacteria, or alternately freezing and thawing phage-infected 

 cells sensitizes the DNA of the infecting virus particles to the action of 

 DNAase. (In the case of adsorption to bacterial ceU-wall debris, the phage 

 DNA is liberated into the medium; in the case of heat killed or frozen-thawed 

 bacteria, the phage DNA enters a host cell whose membrane has been 

 rendered permeable to the DNAase.) Hence, the bulk of the phage protein 

 appears to be relieved of any further function in the intracellular reproduction 

 processes after the proteinaceous tail has made possible the specific adsorption 

 of the virus particle to the bacterial surface and the DNA has been safely 

 "injected" into the interior of the host cell (cf. Chapter 6). 



However, before dismissing the phage protein from further consideration 

 altogether, some mention must be made of that 20 % of the amino acid 

 radioactivity which is not removed from the infected cells in the blendor 

 experiment. Much of this protein fraction probably represents these parts of 

 the phage tails which adhere too firmly to the bacterial cell wall to be shaken 

 loose, for electron microscopic observations show that the materials stripped 

 from the infected cells by blendor treatment are phage ghosts, which are 

 intact except for shortened tails (Levinthal and Fisher, 1953). An indication 

 that, at most, a small portion of this nonremovable 20 % of the phage 

 protein actually enters the infected ceU is that less than 1 % of the parental 

 methionine or lysine reappears in the protein of the progeny phage particles 

 (Hershey and Chase, 1952; French, 1954)— in contrast to the atoms of the 



