212 A. GAREN AND L. M. KOZLOFF 



labeled with radiosulfur (protein specific, mainly in methionine) or radio- 

 phosphorus (DNA specific), and then exposed the cells to a shearing force 

 in a blendor. They found that blending detached from the cells 80 % of the 

 sulfur but only 20 % of the phosphorus of the infecting particle. The ability 

 of the cells subsequently to produce phage offspring was not affected by the 

 treatment. This result indicates that after attachment most of the phage 

 DNA enters the ceU, while most of the phage protein remains extracellular. 

 Electron micrographs support this conclusion. They show empty ghosts 

 attached to the surface of infected cells (see Fig. 6); after blending, the ghosts 

 are found detached (Levinthal and Fisher, 1953). The coat of T2 has aptly 

 been called a microsyringe (Hershey, 1953) designed for the injection of its 

 genetically potent contents into a bacterial cell. 



The 20 % portion of the DNA phosphorus detachable in the blendor 

 appears to be contained largely in intact particles that fail to inject and 

 presumably are noninfective (Hershey and Burgi, 1956), The 20 % portion 

 of the protein sulfur (also protein carbon) that is not detachable is mostly 

 ghost protein that remains attached to the cell surface (Hershey, 1955). 

 There is injected along with the DNA a small amount of non-DNA materials, 

 which include the TCA-soluble components and the internal protein (see 

 Section II). 



Blendor experiments on T5 using P^^-labeled phage (Luria and Steiner, 

 1954) have shown that the DNA of this phage also is injected during infection; 

 injection is slow as compared to T2 and requires addition of Mg"^"*" or Ca'*"+ to 

 the medium. No blendor experiments have been reported for any other 

 phages. Less direct evidence for an injection step in Tl has been obtamed 

 from other kinds of experiments (Christensen and Tolmach, 1955). 



Before proceeding to discuss the steps involved in injection, it may be 

 informative to consider the magnitude of the problem of releasing phage 

 DNA from a protein coat. DNA has the form of a fiber 20 A in diameter. 

 The DNA in T2, amounting to 2 X 10~^^ g., has a total contour length of 

 about 100,000 A and therefore must be considerably folded to fit within the 

 phage head. (The basic substances in the head (see Section II) may serve to 

 reduce the net negative charge of the DNA and thus aid in bringing about 

 the required folding.) The hollow tube in the tail through which the DNA 

 leaves the head is approximately 100 A in diameter and 1000 A in length. It 

 is not known how the DNA is arranged to fit inside the head, but it probably 

 requires careful orientation to pass through the narrow tail. 



V. The Attachment Reaction 

 A. Reversibility 



If attachment of a phage particle leads to infection, the over-all reaction 

 will, of course, be irreversible, since the infecting particle cannot be 



