REPRODUCTION OF VIRUSES: A COMPARATIVE SURVEY 553 



II. Multiplication of Bacteriophage 

 A. The Nature of the Replicating Phage Material 



A tadpole-shaped phage particle attaches itself by the tip of its tail to the 

 bacterial cell wall (Anderson, 1951). After a complex series of mutual inter- 

 actions between phage and cell envelopes (KeUenberger and Arber, 1955; 

 Kozloff e^ al., 1957), the phage particle injects into the cell its DNA, together 

 with some minor constituents (Hershey and Chase, 1952; Hershey, 1955, 

 1957). The protein shell is left at the surface and plays no further role in 

 virus midtiplication. This separation of the viral DNA from the protein shell, 

 which is needed for attachment to cells, explains the "eclipse" of infectivity 

 observed when extracts of newly infected bacteria are tested for ability to 

 infect other cells (Doermann, 1952). 



Following penetration of phage DNA, the infected cell may follow one of 

 two paths,^ depending on the genetic properties of the phage and on the en- 

 vironmental conditions: either the path of virus rephcation in "vegetative" 

 form (Doermann, 1953), followed by virus maturation, cell lysis, and virus 

 liberation; or the path of lysogeny (Lwoff, 1953), in which the cell multiplies, 

 the virus persists in a noninfectious form and, as "prophage," becomes closely 

 and persistently associated with the genetic apparatus of the bacterial cell 

 (Jacob and Wollman, 1957). In the progeny of the lysogenic cells the prophage 

 manifests itself occasionally by shifting to the vegetative form, which multi- 

 plies and produces mature virus and cellular lysis. 



There is direct biochemical evidence that the phage material, both in its 

 vegetative and in its prophage form, consists of DNA. The evidence concern- 

 ing the vegetative form of phage derives mostly from work on coliphage T2. 

 Isotope experiments have shown that in the cells that are going to produce 

 phage there accumulates a pool of specific phage-precursor DNA (Hershey, 

 1956a,b), which is identifiable as phage DNA by its content of the unique 

 pyrimidine (hydroxymethyl) cytosine, instead of cytosine (Wyatt and Cohen, 

 1952). In the pool, the phage-precursor DNA is not associated with any 

 phage-precursor protein related to the proteins of the phage coat (Hershey 

 and Melechen, 1957). Upon maturation, the phage-precursor DNA is removed 

 at random from the pool and then becomes associated with phage-specific 

 proteins. Synthesis of some protein (Cohen and Fowler, 1947; Burton, 1955; 

 Tomizawa and Sunakawa, 1956) and, possibly, also of some specific RNA 

 (Volkin and Astrachan, 1957) is required for the synthesis of phage DNA. 

 These specific RNA and protein maybe necessary intermediates in the replica- 

 tion of DNA. There may actually be a transfer of information from DNA to 



^ Other alternatives, such as abortive infection, or persistence of a nonmultiplying 

 phage element in the cell, will not be considered here, insofar as they do not lead to 

 multiplication. 



