A. D. HERSHEY 



Metabolic Effects of Infection 



Soon after infection, bacteria stop making respiratory and 

 other enzymes (9,51) and fail to respond to inductors of adaptive 

 enzymes (49), as if the metabolic equipment of the cell were 

 frozen at the time of infection. Protein synthesis as a whole 

 continues unabated, however. By contrast, the synthesis of 

 bacterial RNA and DNA stops abruptly, being replaced by the 

 synthesis of viral DNA (6). The latter point has been con- 

 firmed by transferring infected cells into C^'^-glucose, when it is 

 found that no G^^ enters DNA-cytosine (29). The bacterial 

 DNA that was formed before infection disappears afterward, in 

 contrast to bacterial RNA and protein which remain com- 

 paratively inert. Cytological changes suggest that the bacterial 

 nuclei are quickly dispersed (44) . 



These facts have led to the following generalizations. (7) 

 Infection causes a shift from the synthesis of characteristic 

 bacterial constituents to the synthesis of characteristic viral con- 

 stituents, without major substitutions in enzymatic equipment 

 (9). (2) Viral genes replace bacterial genes: infection is 

 parasitism at the genetic level (44). For the present these 

 two ideas are no more than colorful, rather clouded, state- 

 ments of fact but, as Luria (44) has pointed out, it may be useful 

 to look for connections between them. 



Structure of Vegetative Phage 



I have summarized reasons for believing that the sub- 

 stance derived from a single viral particle and directly initiating 

 viral growth consists of one or more linear molecules. If this is 

 so we want to know their number and function, and whether 

 they multiply before or after forming more complex struc- 

 tures. Structure can be got at in several ways besides the 

 attempt to isolate vegetative phage, and some of these are 

 surveyed below. About function only one question can be asked 

 in advance of information about structure. It is concerned with 



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