FATE OF INFECTING PHAGE PARTICLES 215 



As discussed in Chapter XVII, superinfection breakdown is 

 probably related to but does not cause genetic exclusion of 

 superinfecting phage. Similarly, it seems clear that superin- 

 fection breakdown results from the action of bacterial deoxyri- 

 bonuclease, but it is not likely that it depends simply on the 

 activation of this enzyme after infection, because T3 activates 

 enzyme but does not activate the breakdown mechanism. 



Pardee and Williams (1952, 1953) found that the activity of 

 bacterial deoxyribonuclease increases after infection with T2 and 

 T3. KozlofF (1953) showed that the effect is due to the destruc- 

 tion of a specific inhibitor of the enzyme, probably part of the 

 bacterial ribonucleic acid. 



The role of the enzyme in superinfection breakdown is indi- 

 cated by the fact that streptomycin and low magnesium concen- 

 trations interfere with breakdown and with the action of the 

 enzyme (Graham, 1953). 



Superinfecting phage is treated differently from the primary 

 infecting phage in three ways. The superinfecting phage, only, 

 is subject to a powerful genetic exclusion, to partial interference 

 with injection, and to complete breakdown of the injected DNA. 

 Inhibition of deoxyribonuclease prevents breakdown but does 

 not affect exclusion or injection. The exclusion may mean that 

 injection is not only incomplete but also abnormal in other, un- 

 known, ways. The fact that the breakdown products do not re- 

 main in the cells also suggests this conclusion. If so breakdown, 

 though requiring active deoxyribonuclease, probably depends 

 primarily on the hypothetical, abnormal kind of injection. In 

 any case the breakdown as such does not influence the outcome 

 of the infection in any discernible way (Graham, 1953; Hershey, 

 Garen, Fraser, and Hudis, 1954). 



Also following primary infection, a small amount of the phage 

 DNA, not exceeding 5 per cent under optimal conditions, is 

 broken down to low molecular weight material by the intrabac- 

 terial deoxyribonuclease. This may reflect some side reaction 

 to which only a few of the phage particles are subject, rather than 

 a necessary consequence of the infectious process itself. Further 



