BIOLOGICAL ROLE OF DEOXYPENTOSE NUCLEIC ACIDS 463 



the DNA may even be "injected" into the host cell through the phage tail 

 structure. 



Other observations made with labeled phage support this general picture. 

 In all cases, protein, recognized by its S^* content (and sometimes by its 

 amino acid composition), is the material which combines with specific 

 antiserum or with the surface of susceptible normal or with heat-killed or 

 disintegrated bacteria and becomes easily sedimentable. These properties 

 are characteristic of the protein fraction whether it is in intact phage, in 

 osmotically produced ghosts, in the material stripped from infected cells,, 

 or in the material not stripped off but recovered later with the bacterial 

 debris. The DNA, on the other hand, behaves differently at different sites. 

 It is not attacked by deoxyribonuclease either in intact phage or during or 

 after it is transferred to the infected bacterium. Only in a system disturbed 

 by some intervention which also disrupts phage reproduction has the DNA 

 been made vulnerable to nuclease. Before infection, damage to phage mem- 

 branes will expose DNA; after infection disintegration of bacterial mem- 

 branes will suffice.^^^'*^ When adsorbed on bacteria previously heat-killed, 

 frozen and thawed, or otherwise disintegrated, the protein of normal phage 

 is bound, but the DNA becomes nuclease- vulnerable and can be readily 

 extracted. It may have been extruded or injected upon, instead of into, the 

 damaged cells. A somewhat similar accident befalls the DNA of certain T 

 phages added to already infected cells. In such a case, the superinfecting 

 phage begins to disintegrate spontaneously, giving off P-containing frag- 

 ments,^" and fails to introduce its phage characteristics into the progeny, ^^* 

 although the DNA may not become markedly susceptible to external de- 

 oxyribonuclease. With certain other T phages, there is little breakdown of 

 late-coming phage but its phosphorus still is not taken into new phage. ^^^ 



On the other hand, when it is the phage which is biologically inactivated 

 (by formaldehyde), adsorption on the cells does not result in release of 

 DNA either into the cell or in a state susceptible to nuclease from without.^** 

 Infection is not established and, in this case, mechanical stripping of the 

 cell-phage complex removes DNA along with the phage coats. 



Intact T phage, then, displays all the properties of its protein but not 

 those of its DNA; after physical damage both components are revealed. 

 Phage DNA is therefore considered to be free or very loosely combined 

 inside a protective membrane comprised of the phage protein. At infection 

 the DNA passes into the inside of the bacterial cell ; the protein remains at 



1" A. Siegel, S. J. Singer, and S. G. Wildman, Arch. Biochem. and Biophys. 41, 278 



(1952). 

 1" S. M. Lesley, R. C. French, A. F. Graham, and C. E. van Rooyen, Can. J. Med. 



Set. 29, 128 (1951). 

 •68 R. Dulbecco, /. Bacterial. 63, 209 (1952). 

 1" A. F. Graham, Ann. inst. Pasteur 84, 90 (1953). 



