26 S. S. COHEN 



essential to the production of a viral-specific intermediary metabolite. 

 Although it has been shown that the early protein synthesis, which occurs 

 before DNA synthesis in the eclipse period, is essential to virus production 

 (Cohen and Fowler, 1947) and to a subsequent DNA synthesis (Burton, 1955; 

 Tomizawa and Sunakawa, 1956; Hershey and Melechen, 1957), it must also 

 be noted that such early protein synthesis is also important for phages which 

 do not contain HMC (Miki and Matsushita, 1956). Nevertheless, it has 

 recently been shown that the appearance of the deoxycytidylic hydroxy 

 methylase induced by infection with T-even phages containing their DNA 

 does require protein synthesis, tending to support hypotheses of the "b" 

 type (Flaks and Cohen, 1958). 



2. The other line of evidence suggesting a host synthesis of viral polymers 

 is of a more biological character and is more complete. It derives from the 

 data on lysogeny and the behavior of the prophage. The following attributes 

 of lysogenic systems may be taken to suggest that the enzymes of the host 

 are involved in the synthesis of viral polymers and that the genetic material 

 of the lysogenic viruses may act to tie up these synthesizing capacities. 



a. The presence of a prophage produces immunity to supermfection by 

 a mutant temperate phage (Lwoff, 1953), suggesting competition for a 

 specific synthetic mechanism, the numbers of which are limited by the host. 



b. The cytological basis of the above is indicated by the findings that 

 (1) the prophage is mcorporated into the bacterial genome; (2) it exists in 

 the same number as do the bacterial nuclei; (3) the prophage is locaHzed at a 

 specific site on (although perhaps not in) the bacterial chromosome; and 

 (4) a prophage is duphcated at the rate of duphcation of other genetic loci 

 (Jacob and WoUman, 1957). 



c. Analysis of the mechanism of induction by ultraviolet irradiation of 

 lysogenic bacteria containing two different prophages has led to the con- 

 clusion that prophage development results from an indirect effect mediated 

 through the bacterium, rather than from a direct change in the prophage 

 (Jacob, 1954). 



d. In certain lysogenic systems, a partial genetic homology may exist 

 between a phage and its host (Garen and Zinder, 1955). After infection, host 

 material may replace homologous phage substance damaged by irradiation. 

 Such a reactivation cannot occur if the host material has been similarly 

 damaged. Some of the virulent phages, such as Tl and T3, may also undergo 

 cellular reactivation after sustaining irradiation damage. One may imagine 

 that the genetic homology reflects a chemical identity of the polymers 

 involved and that the enzymes of the host are capable of synthesizing 

 polymers which are specific and characteristic of host and phage alike. 



In addition, we may note that photoreactivation of ultraviolet-irradiated 

 viruses (including the chemically unique T-even set) is effected by the 



