A. D. HERSHEY AND MARTHA CHASE 55 



the phage DNA in a form not precipitable by antiserum and not adsorbable 

 to bacteria. The sulfur-containing protein of the phage particle evidently 

 makes up a membrane that protects the phage DNA from DNase, comprises 

 the sole or principal antigenic material, and is responsible for attachment of 

 the virus to bacteria. 



2. Adsorption of T2 to heat-killed bacteria, and heating or alternate freezing 

 and thawing of infected cells, sensitize the DNA of the adsorbed phage to 

 DNase. These treatments have little or no sensitizing effect on unadsorbed 

 phage. Neither heating nor freezing and thawing releases the phage DNA 

 from infected cells, although other cell constituents can be extracted by these 

 methods. These facts suggest that the phage DNA forms part of an organized 

 intracellular structure throughout the period of phage growth. 



3. Adsorption of phage T2 to bacterial debris causes part of the phage 

 DNA to appear in solution, leaving the phage sulfur attached to the debris. 

 Another part of the phage DNA, corresponding roughly to the remaining half 

 of the DNA of the inactivated phage, remains attached to the debris but can 

 be separated from it by DNase. Phage T4 behaves similarly, although the 

 two phages can be shown to attach to different combining sites. The inactiva- 

 tion of phage by bacterial debris is evidently accompanied by the rupture of 

 the viral membrane. 



4. Suspensions of infected cells agitated in a Waring blendor release 75 per 

 cent of the phage sulfur and only 15 per cent of the phage phosphorus to the 

 solution as a result of the applied shearing force. The cells remain capable of 

 yielding phage progeny. 



5. The facts stated show that most of the phage sulfur remains at the cell 

 surface and most of the phage DNA enters the cell on infection. Whether 

 sulfur-free material other than DNA enters the cell has not been determined. 

 The properties of the sulfur-containing residue identify it as essentially un- 

 changed membranes of the phage particles. All types of evidence show that 

 the passage of phage DNA into the cell occurs in non-nutrient medium under 

 conditions in which other known steps in viral growth do not occur. 



6. The phage progeny yielded by bacteria infected with phage labeled 

 with radioactive sulfur contain less than 1 per cent of the parental radioactiv- 

 ity. The progeny of phage particles labeled with radioactive phosphorus con- 

 tain 30 per cent or more of the parental phosphorus. 



7. Phage inactivated by dilute formaldehyde is capable of adsorbing to 

 bacteria, but does not release its DNA to the cell. This shows that the inter- 

 action between phage and bacterium resulting in release of the phage DNA 

 from its protective membrane depends on labile components of the phage 

 particle. By contrast, the components of the bacterium essential to this inter- 

 action are remarkably stable. The nature of the interaction is otherwise un- 

 known. 



8. The sulfur-containing protein of resting phage particles is confined to a 



103 



