234 BACTERIOPHAGES 



sented in reviews by Cohen (1949, 1953b, 1956) and by Putnam 

 (1952) and the topic has been briefly considered in a number of 

 general reviews of phage work such as that by Benzer, Delbriick, 

 Dulbecco, Hudson, Stent, Watson, Weidel, Weigle, and Woll- 

 man (1950) and one by Putnam (1953). The role of the enzy- 

 matic constitution of the host cell as a factor in the nutritional 

 environment of the reproducing phage has been discussed by 

 Cohen (1952) and by E. A. Evans, Jr. (1954). 



1. Requirements for Adsorption and Penetration 



The role of the ionic environment in phage adsorption has been 

 discussed extensively in Chapter X and will be briefly summa- 

 rized here. Adsorption of phage to host cell involves at least two 

 separate steps, the first step being a reversible attachment of 

 phage particle to host cell and the second step being irreversible. 

 The second step involves liberation of phage DNA from the pro- 

 tein membrane and under appropriate conditions this DNA 

 penetrates into the host cell interior. For a particular phage- 

 host cell system there is an optimum salt concentration at which 

 the rate of attachment is maximal. The optimum salt concentra- 

 tion for monovalent cations diff'ers from that for divalent cat- 

 ions in instances where either is effective. The cationic environ- 

 ment also controls the rate of the second step and for some phages 

 the cationic requirement is rather specific. A surprisingly large 

 proportion of the phages which have been examined have a more 

 or less specific requirement for calcium ion. In many cases this 

 requirement is known to involve a step subsequent to adsorption 

 and in the case of phage T5 the step is definitely penetration of 

 the phage DNA into the host cell. In addition to the ionic re- 

 quirements just mentioned, there is one case of a relatively 

 specific requirement for an organic cofactor for adsorption, the 

 tryptophan-requiring strains of phage T4 which have been dis- 

 cussed in Chapter X. It would be surprising if this were a 

 unique case. 



The very extensive literature dealing with the ionic cnxiron- 

 ment required for adsorption or penetration would make pretty 



