33. NUCLEIC ACIDS OF THE BACTERIAL VIRUSES 211 



present at the time of irradiation and DNA made subsequently. Both 

 irradiated and nonirradiated DNA can appear in either infective or non- 

 infective particles. Nonirradiated, labeled DNA in noninfective particles 

 can later be separated from irradiated DNA by a second generation experi- 

 ment in which the noninfective particles are employed in a mixed infection 

 with a large multiplicity of unlabeled live particles. Among the progeny of 

 such a cross, 90% of the transferred P 32 is found in infective particles. 

 In the reciprocal experiment (with noninfective particles containing labeled, 

 irradiated DNA) 50% of the transferred P 32 is found in infective particles, 

 while 50 % persists in noninfective particles. 



These experiments seem to indicate that the DNA irradiated within the 

 cell is incorporated into the mature phage without any process of molecular 

 degradation and loss of identity. 



The conservation of ultraviolet lesions per genome after irradiation was 

 only demonstrated in the instance (11 hits) that subsequent DNA synthesis 

 was totally blocked. It would be very interesting to know if this conserva- 

 tion would be observed after a lesser ultraviolet dose which permitted post- 

 irradiation DNA synthesis. 



Additional evidence that the phage DNA which accumulates in the pres- 

 ence of chloramphenicol is specific and functional, 106 is provided by experi- 

 ments with the mutagen 5-bromouracil. If infected cells which require 

 thymine (either as bacterial mutants or by exposure to sulfanilamide) are 

 instead provided with 5-bromouracil, this pyrimidine is extensively incor- 

 porated into the phage DNA in the place of thymine. 110 The mutation 

 frequency among such phage may be two orders of magnitude higher than 

 is usually found. 111 



If, specifically during the time of DNA synthesis in the presence of 

 chloramphenicol, thymine-requiring cells are supplied with 5-bromouracil 

 and this is then removed at the time of chloramphenicol removal, the prog- 

 eny phages are found to contain more than an order of magnitude increase 

 in incidence of mutation. 112 ' 113 Since, presumably only the DNA made in 

 the presence of chloramphenicol (and its putative progeny) could have 

 been affected by the 5-bromouracil, this DNA must be functional. 



3. Biochemistry of T-Even Phage Infection 



a. Origin of Phage DNA Components 



The biochemistry of the synthesis of phage DNA has not been developed 

 in detail. The phosphorus, as previously indicated, comes to some extent 



110 D. B. Dunn and J. D. Smith, Nature 174, 305 (1954). 



111 R. M. Litman and A. B. Pardee, Nature 178, 529 (1956). 



112 S. Brenner and J. D. Smith, Virologij 8, 124 (1959). 



113 R. M. Litman and A. B. Pardee, Virology 8, 125 (1959). 



