THE INITIATION OF BACTERIOPHAGE INFECTION 233 



process with several properties of actomyosin. At present it would be un- 

 warranted to label the proximal tail protein as a form of actomyosin, especially 

 in view of lack of information about the behavior of the tail protein toward 

 ATP, The exact function of the contractile step is not yet apparent. It might 

 be proposed that the contraction of the proximal protein aids in penetrating 

 the ceU waU by forcing the tail core through the cell membrane. The tail core 

 may then come out of the tail and perhaps remain inside the cell. On the 

 other hand, the contractile step may be more like the opening of a sphincter 

 to permit the DNA to pass through the tail. 



4. Release of Cell Wall Material 



An enzyme in the phage tail removes a part of the cell wall that is respons- 

 ible for its rigidity. This enzyme apparently acts only on the limited area of 

 the ceU wall which is in contact with the enzyme. Possibly the contractile 

 step ensures close contact of the insoluble enzyme with the insoluble substrate. 



5. Injection of Phage DNA 



The forces involved in injecting the phage DNA are not known. The 

 decrease in the viscosity barrier when the proximal tail protein contracts 

 does not permit the immediate release of the DNA. A chemical triggering 

 agent would seem to be necessary. In model experiments DNA is released 

 from the head by a variety of compounds which contain primary amino 

 groups. The best candidate for this role during invasion is the material 

 released by the action of the tail enzyme on the ceU wall, since it contains 

 free amino groups. 



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