INTEACELLULAR MULTIPLICATION OF BACTERIAL VIRUSES 265 



phage DNA, which do enter the cell and which, as will be seen below, are 

 transferred efficiently to the DNA of the progeny phages. In fact, Hershey 

 (1955) has been able to fix an upper hmit of 3 % for the amount of total phage 

 protein which could enter the infected cell and play some role in the course of 

 intracellular phage reproduction. An "acid-soluble" peptide has, however, 

 been identified, which is injected into the host cell by the parental phage but 

 not transferred to the progeny. This peptide amounts to about 2 % of the 

 total phage protein and contains no sulfur, being composed mainly of lysine, 

 glutamic acid, and aspartic acid. The phage also injects some polyamines 

 which are transferred as such to the progeny particles (Hershey, 1957b; Ames 

 et d. 1958). 



The discovery that the DNA, and not the protein, of the infecting phage 

 enters the host cell at the moment of infection thus clearly shows that it must 

 be the DNA which is the carrier of the genetic continuity of the virus particle, 

 i.e., the germinal substance of the extracellular, resting phage. (The 20 % of 

 the phage DNA found to be strippable in the blendor experiment probably 

 represents a minor fraction of the phage population, which does not infect the 

 host properly and which fails to inject its nucleic acid core (Hershey, 1953b).) 

 Such an assignment of a genetic role to the viral DNA is in perfect harmony 

 with notions derived from bacterial transformation experiments, in which 

 the hereditary factors of a donor baciUus are transferred to the genome of a 

 receiver cell through the exclusive vehicle of DNA molecules (Avery et al., 

 1944). The release of the phage DNA from its protein envelope at the very 

 moment of infection, furthermore, now readily accounts for the existence of 

 an ecHpse period at the early stages of intracellular virus development, when 

 no infective particles whatsoever can be recovered by induced lysis of the host 

 cell. For having just been divested of its attachment and injection organs, the 

 DNA of the infecting phage is naturally unable to gain entrance into any 

 further bacterial cells to which it may be presented in the infectivity test. It is 

 actually not excluded a priori that phage DNA, like bacterial DNA in trans- 

 formation experiments, can gain entrance into and infect bacterial cells 

 directly. Experiments to test this possibility have generally led to negative 

 results (Cohen, 1947), although it has been claimed that phage DNA can 

 infect bacteria whose ceU wall has been weakened by various treatments 

 (Spizizen, 1957; Fraser et al, 1957). 



B. Transfer of the Parental Nucleic Acid 



1. Extent and Specificity of Transfer 



In 1950, Putnam and Kozloff invented an experiment directed toward the 

 question of how much, if any, of the substance of the DNA introduced into 

 the host cell by the infecting bacteriophage reappears among the DNA of its 



