204 A. GAREN AND L. M. KOZLOFF 



1. Attachment 232 



2. Alteration of the Tip of the Phage Tail 232 



3. Contraction of the Phage Tail 232 



4. Belease of Cell Wall Material 233 



5. Lijection of Phage DNA 233 



References 233 



I. Introduction 



In the course of their growth cycle, phages undergo a metamorphosis from 

 a metabohcally inert extracellular form to an active intracellular form. The 

 pathway of this transition provides the subject matter for the present chapter. 

 The central theme is the remarkable process of injection, which serves to 

 introduce the hereditary material of the phage into a bacterial host cell. 

 Sections II and III deal with the composition and organization of the com- 

 ponents involved in injection, namely, the extracellular phage particle and 

 the exterior surface of the bacterial ceU. In Section IV are described the char- 

 acteristics of the injection process. Sections V and VI are concerned with 

 physical-chemical and genetic properties of the initial attachment reaction 

 between a phage and a ceU (this reaction being of major significance in deter- 

 mining the extraordinary selectivity of the phage-host relationship). Kecent 

 chemical and morphological studies bearing on the mechanism of injection 

 are reviewed in the final section. 



A review of work on phage infection must depend heavily on information 

 obtained from a single system: phage T2 and its host Escherichia coli. Conse- 

 quently, many of the facts and inferences may not have validity for other 

 phages. It is already apparent that some of the properties of T2 are not found 

 in other unrelated phage strains that have been studied. Nevertheless, we 

 believe that in considering certain important features of phage morphology 

 and behaviour, in particular the chemical and structural relationship of the 

 genetic and nongenetic materials and their respective fates during infection 

 (see Section IV), T2 can serve as a model phage. 



The organization of the present chapter has been influenced by the 

 recent appearance of two excellent reviews dealing with many of the topics 

 concerned (Hershey, 1957a; Tolmach, 1957). Therefore we have not found it 

 necessary to cover all points that may be pertinent to the subject at hand, 

 and equal attention has not been given to those covered. 



II. Morphology and Composition of Bacteriophages 

 A. Size and Shape 



Many properties of phages have been measured in order to determine 

 particle size, notably, rates of diffusion and sedimentation (Putnam, 1953), 



