234 ROBERT L. SINSHEIMER 



members of the T2, T4, T6, and T5 group is extremely resistant to ir- 

 radiation; it is very much more resistant than the capacity of the irradi- 

 ated cells to form colonies. The capacity of E. coli B cells to support 

 the growth of Tl or T3 is much more sensitive to ultraviolet irradiation, 

 although still not as sensitive as the colony forming ability. Apparently 

 phage of the latter group require a considerably greater contribution to 

 the function of the phage-bacterium complex from the host genome than 

 do phages of the T2, T4, T6, and T5 group. 



The idea of a partial genetic homology between phages of the Tl, T3, 

 and T7 group and the host bacterium has been introduced to explain the 

 relatively low ultraviolet sensitivity of members of this class (explained 

 by a reconstitution of active phage during infection by recombination be- 

 between an ultraviolet damaged phage genome and an undamaged host 

 genome), and to interpret the effect of ultraviolet irradiation of the host 

 cell prior to infection, upon the apparent survival of phage irradiated with 

 ultraviolet light prior to infection. 6, 183 ' 184 



With members of the Tl, T3, T7 group the apparent survival of phage 

 after a given ultraviolet dose is strongly dependent on the exposure of 

 the host cells to ultraviolet irradiation prior to infection (Fig. 7). With 

 a light dose of ultraviolet irradiation to the cells, the apparent phage 

 survival is markedly increased. This is interpreted as a result of a "stimu- 

 lus to recombination" by ultraviolet irradiation of the host, giving rise 

 to an increased number of phage reactivations by recombination. This 

 phenomenon of "ultraviolet reactivation" is not observed with members 

 of the T2, T4, T6, and T5 group. 



With increasing ultraviolet dose to the bacterial cell, however, the ap- 

 parent phage survival, after a given ultraviolet dose, falls off extremely 

 rapidly. Indeed, if the survival of ultraviolet-treated phage of this group 

 is measured on heavily ultraviolet irradiated bacteria, their apparent sen- 

 sitivity per milligram of DNA becomes comparable to that of members 

 of the other phage group, T2, T4, T6, and T5. 



These data are interpreted to mean that the apparent low sensitivity 

 of members of this group to ultraviolet irradiation when tested upon nor- 

 mal bacteria is due to a considerable amount of phage reactivation by 

 recombination with homologous regions of the bacterial genome. With low 

 doses of ultraviolet light to the host cell, recombination and thus reacti- 

 vation are facilitated. With increasing dose the region of the bacterial 

 genome necessary for reactivation becomes damaged, and the ultraviolet 

 sensitivity of the phage increases to the level it would have had in the 

 absence of a reactivation process. 



184 A. Garen and N. D. Zinder, Virology 1, 347 (1955). 



