33. NUCLEIC ACIDS OF THE BACTERIAL VIRUSES 239 



result appears to demonstrate that the initial prophage structure, if it 

 is DNA, is not composed of the DNA molecules of the parental phage. 



Siminovich 185 has indicated that DNA synthesis continues unaltered 

 upon infection with lambda phage. 



That the phage particle must carry out certain functions before lyso- 

 genization has been shown by the studies of temperate phage cooperation 

 by Levine 193 and by Kaiser. 195 Mutant phages of P22 or of X were obtained 

 with reduced frequencies of lysogenization (clear mutants). These could 

 be grouped into phenotypic classes according to the frequency of lyso- 

 genization. If mixed infections with phages from different phenotypic 

 classes were carried out, a high frequency of lysogenization, characteris- 

 tic of the wild-type, phage, could be obtained, even though in many cases 

 the prophage which became established was not a recombinant but one 

 of the parental types. This result indicates a cooperative type of action 

 preceding lysogenization analogous to the cooperation between phages 

 mutant in the A and B cistrons of the rll region of T2 in infection of 

 K12 (X), as demonstrated by Benzer. 136 



Also, the new somatic antigen, which develops in Salmonella typhimurium 

 infected by phage P22, can be detected within 6 to 8 minutes after infec- 

 tion. 188 At this time the cells become immune to superinfection with phage 

 P22. It is of interest that even though, through segregation, many of 

 the originally infected cells throw off nonlysogenic progeny, this immu- 

 nity persists among such progeny for 8-10 generations. This result indi- 

 cates that the direct presence of the prophage is not essential for immu- 

 nity. 191 



Studies of the changes in the ultraviolet sensitivity of the phage-bac- 

 terium complex during both the lytic and the lysogenic responses following 

 infection by a temperate phage have been made by L. E. Bertani. 196 In 

 either case, ultraviolet sensitivity, which is initially similar to that of the 

 free phage, decreases during the first 15-20 minutes by a factor of 5-10. 

 In the lytic complexes the sensitivity then remains at this low level until 

 lysis. In the lysogenic complexes, however, the sensitivity increases after 

 15 minutes of infection, reaching a maximum which is greater than that 

 of free phage, or of established lysogenic cells, at about 60 minutes after 

 infection. Not until 2 hours after infection do the lysogenized cells ac- 

 quire the ultraviolet resistance characteristic of an established culture. 



4. Nature of the Prophage 



The most nearly definitive data upon the nature and size of the pro- 

 phage may be derived from experiments of Stent et al. 197 When a bac- 



195 A. D. Kaiser, Virology 3, 42 (1957). 



196 L. E. Bertani, Virology 7, 92 (1959). 



197 G. S. Stent, C. R. Fuerst, and F. Jacob, Compt. rend. acad. set. 244, 1840 (1957). 



