THE INITIATION OF BACTEEIOPHAGE INFECTION 213 



reconstituted after the DNA has been released from the protein coat. Other 

 irreversible changes preceding the release of DNA also occur (see Section VII). 

 However, when the attachment of Tl and T2 occurs under certain conditions 

 of temperature, ionic environment, or pretreatment of the cells, the attached 

 particles can readily be eluted from the cell surface (Garen and Puck, 1951; 

 Garen, 1954; Christensen and Tolmach, 1955). Since neither the phage nor the 

 cell appears to be changed after this kind of encounter, the reaction is called 

 reversible. Several properties of the reaction indicate that ionic bonds, but 

 not covalent bonds, are involved. 



On the basis of results obtained primarily with Tl (Tolmach, 1957) it has 

 been concluded that reversible attachment serves as the first in a sequence 

 of steps on the pathway to infection. Under suitable conditions, reversible 

 attachment would rapidly be followed by irreversible changes culminating in 

 the injection of phage DNA: 



Phage + Cell ^Reversible Attachment -> Irreversible Attachment > Iiifection 



A different interpretation has also been proposed (Hershey, 1957a), in which 

 reversible attachment is considered, not as one of the steps of infection, but 

 rather as an independent competitive reaction: 



Reversible Attachment 



/ 

 Phage + Cell 

 \ 



Irreversible Attachment > Infection 



The evidence pertaining to the role of reversible attachment in the process 

 of phage infection has been critically discussed in two recent reviews 

 (Tolmach, 1957; Hershey, 1957a). 



B. Influence of Temperature 



The velocity of phage attachment to host cells is almost as rapid at as 

 at 37°C., but other characteristics of the reaction may vary markedly in this 

 temperature range. With Tl and T7, attachment changes from an irreversible 

 reaction at 37°C. to one that is largely reversible at 0°C., which indicates 

 that the first of the irreversible attachment reactions has a high temperature 

 sensitivity and therefore involves covalent bond formation (Garen and Puck, 

 1951; Garen, 1954; Mackal and Kozloff, 1954; Christensen and Tolmach, 

 1955). With T2, in contrast to Tl and T7, attachment is irreversible at as 

 well as at 37°C. However, at the lower temperature, attachment of T2 does 

 not lead to infection. Most of the particles attaching at 0°C. eject their DNA 

 into the medium, which accounts for the irreversibility of the reaction, and 

 the cells remain viable (Puck, 1953; Adams, 1955). Evidently the mechanism 

 for ejecting phage DNA from the protein coat after attachment has occurred 



