THE INITIATION OF BACTERIOPHAGE INFECTION 231 



core) has been widened from about 100 to 140 A. Since the viscous drag in 

 passing a solution containing a lughly asymmetrical molecule such as DNA 

 (diameter about 25 A) through a narrow tube is directly proportional to the 

 length and inversely proportional to the fourth power of the diameter of the 

 tube, these morphological changes reduce the viscosity barrier, i.e. the flow 

 time, by about 8-fold. The properties of T2 altered by Cd(CN)-3 or by cell 

 walls support the concept that viscosity is at least one barrier to DNA 

 release. In saline, altered phage do not measurably release their DNA at 

 10-15T.; at 37°C. the DNA is very slowly released, but at 44°C. the DNA 

 is released very rapidly. It is well known that an increase in temperature 

 greatly lowers the viscosity of DNA solutions. 



Since the injection of phage DNA into the host cell occurs rapidly at 37°C., 

 special devices would seem necessary to overcome, not only the viscosity 

 barrier, but also the attractive forces between the DNA and phage protein. 

 It has been found, for instance, that at neutral pH's free phage DNA and 

 protein spontaneously aggregate (Kozloff and Lute, 1957b). In considering 

 the many steps in the invasion mechanism where one reaction controls or 

 initiates the succeeding reaction, it might be expected that DNA release 

 would be chemically controlled and that some compounds would initiate 

 DNA release. In the model experiments using Cd(CN)~3-altered T2, it was 

 found that various compounds containing primary amino groups, such as 

 glucosamine, and various amino acids, such as lysine, caused the rapid 

 release of the phage DNA from its protem covering. The release occurred 

 optimally at pH 8.75-9.0 and appeared to be an all-or-none phenomenon 

 (Fig. 11). It is attractive to consider the action of the various compounds 

 which cause the DNA release as being analogous to the action of similar 

 agents produced during invasion. Almost aU of the compounds which cause 

 DNA release are found in the cell wall. It can be suggested that the products 

 formed by the action of the phage tail enzyme on the host cell waU might be 

 the agents which trigger the release of the DNA into the host ceU. It is 

 worth noting that the products of cell wall digestion ( Weidel and Pringosigh, 

 1957) have free amino groups and include glucosamine, one of the most 

 active agents in causing DNA release from altered phage, as well as muramic 

 acid, diaminopimelic acid, glutamic acid, and alanine. 



D. Summary of the Steps of Invasion 



The sequence of reactions that appear to be involved in invasion by T2 is 

 summarized diagrammatically in Fig. 12. It is perhaps unnecessary to point 

 out that the evidence is not equally strong for each step. This summary may 

 serve as a guide to further experimentation which undoubtedly will modify 

 the present views on the subject. 



