33. NUCLEIC ACIDS OF THE BACTERIAL VIRUSES 231 



Unlike the observations made with the T-even phages, the DNA synthesis 

 during T5 infection appears to require a continuing protein synthesis 

 (Fig. 6). Addition of chloramphenicol after 10 minutes will completely 

 block the development of DNA synthesis. Addition of chloramphenicol 

 at any time during the stage of DNA synthesis will cause a complete 

 halt after a lag of about 5 minutes. 



Also, as with the T-even phages, infection with To appears to confer 

 upon thy mine-requiring cells the ability to synthesize thymine. 173, 175 This 

 was demonstrated by an increase in DNA and specifically in thymine, 

 in To infected cells, in a host strain normally requiring thymine, and 

 upon infection in a thy mine-free medium. Additional evidence was pro- 

 vided by the similarity of the burst size of T5 in such cells in media with 

 or without thymine, by incorporation of label from aspartic acid-C 14 or 

 from serine-3-C 14 into thymine in such cells during To infection (and the 

 absence of such incorporation in uninfected cells), and by a marked de- 

 crease in incorporation of exogenous 2-thymine-C 14 into the DNA of such 

 cells upon To infection. 



By the use of a reverted thy mine-requiring mutant in which the syn- 

 thesis of DNA in the absence (but not in the presence) of exogenous thy- 

 mine was strongly temperature dependent in the region 37°-42°, Craw- 

 ford 173 has demonstrated that the thymine-synthetic ability acquired upon 

 infection appears to be distinct from that of the host in that it does not 

 in this case have a temperature dependence in the 37°-42° range. 



Romberg et al. 77 have shown that the levels of thymidylic, deoxycyti- 

 dylic, and deoxyguanylic acid kinase increase some ten- to fortyfold during 

 T5 infection, while the initially high level of deoxyadenylic acid kinase 

 increases twofold. These increases in activity begin to appear about 10 

 minutes after infection. Similar increases of thymidylic and deoxyguanylic 

 acid kinases are observed during infection with T-even phages. 



As with the T-even phages 176 a rise in deoxyribonuclease activity of 

 bacterial homogenates is observed shortly after T5 infection. However, 

 this may have little to do with the breakdown of host DNA as addition 

 of citrate is observed to inhibit the rise in deoxyribonuclease activity 

 without influencing the course of DNA degradation. 



2. Infection with Bacteriophages Tl, T3, and T7 



Bacteriophages Tl, T3, and T7 have a similar content of DNA (ap- 

 proximately one-third the DNA content of T2) which is, in each case, 

 composed of similar proportions of the usual purines and pyrimidines 

 (Table I). They have similar radiobiological properties (vide infra) and, 



176 L. V. Crawford, Biochim. et Biophys. Ada 30, 428 (1958). 



176 R. E. Kunkee and A. B. Pardee, Biochim. el Biophys. Acta 19, 236 (1956). 



