REPRODUCTION OF VIRUSES: A COMPARATIVE SURVEY 557 



coliphages of the T2 group, wliicli contain the liydroxymethylated nucleo- 

 tide (Wyatt and Cohen, 1952). The enzyme is clearly required for synthesis 

 of phage DNA itself. Although the enzyme may conceivably be present in 

 inactive form in the bacteria and be activated by phage infection, as in the 

 case of a bacterial deoxyribonuclease (Pardee and Williams, 1952; Kozloff, 

 1953), it seems more probable that the enzyme is synthesized anew under the 

 genetic control of the incoming phage DNA. Similar mechanisms may under- 

 lie the restoration or expansion of thymine synthesis in an almost thymineless 

 bacterial strain following infection with phage (Earner and Cohen, 1955). 



3. Syntheses Related to Phage Maturation 



If we accept the concept of phage DNA acting as genetic material in inte- 

 gration with the cell genome and controlling heterocatalj^ically a number of 

 biosynthetic processes, it becomes natural to consider also the proteins of the 

 mature phage particles as special products of the functional activity of the 

 phage genome. 



The proteins of a phage particle comprise a variety of antigenically 

 distinct fractions (Lanni and Lanni, 1953), some of which are probably 

 active enzymatically (Brown and Kozloff, 1957). Some are located in the head 

 of the phage, others, in the tail. The tail proteins include the organ of phage 

 attachment to the cell. When a bacterium is infected with two related phages, 

 whose tail proteins differ in antigenic specificity or in requirements for ad- 

 sorption cofactors (Anderson, 1945), the progeny particles exhibit "pheno- 

 typic mixing" (Novick and Szilard, 1951). That is, the specificity of the tail 

 proteins may correspond, not to the genetic characteristics of the phage 

 particle that carries them, but to the characteristics of the other phage type 

 that was growing in the same ceU, or to a mixture of the two. The association 

 between genetic and phenotypic properties is almost random (Streisinger, 

 1956; Brenner, 1957). This indicates that the two kmds of tail protein are 

 synthesized "at large" in the infected cells and are then utilized, as available, 

 in assembling the coats of the maturing phage, in the same way as gene 

 products controlled by different allelic genes may be utilized side by side in a 

 heterozygous ceU, or as gene products controlled by genetically different 

 nuclei in a heterocaryotic cell or mycelium. 



Other specific proteins, besides those destined to become part of the 

 mature particles, are produced in the process of phage maturation. These 

 include a number of agents that act enzymatically to dissolve the surface 

 layers of bacteria, some digesting capsidar polysaccharides (Adams and 

 Park, 1957), others attacking the bacterial cell wall (Huppert and Panijel, 

 1957; Murphy, 1958; Jacob and Fuerst, 1958). Such enzymes play a role 

 in bacterial lysis, in the release of the newly formed phage, and in 



