214 BACTERIOPHAGES 



suited in liberation of 56 per cent, 53 per cent, and 74 per cent, 

 respectively, of the added P'''-. 



8. Superinfection breakdown of 'r2/'+ phage occurs after pri- 

 mary infection with T2, T4, and T6 phage, r or r+, and with T5 

 but does not occur after primary infection with Tl, T3, or T7. 

 Superinfection with r phages follows the same pattern as with 

 r+ phages but the amount of breakdown is less (30 per cent). 

 Primary infection with T3 followed by superinfection with 

 T2r+, followed still later by superinfection with labeled T2r+ does 

 not lead to superinfection breakdown. Primary infection with 

 T2 or T7 does not result in breakdown of superinfecting T7. 



9. Superinfection breakdown does not occur in heat-killed 

 bacteria but does occur in bacteria killed by ultraviolet light. 

 Superinfection breakdown is prevented if cyanide is added to the 

 bacteria before the primary infection, but is not prevented if 

 cyanide is added 5 minutes after primary infection and before 

 superinfection. One may recall that in cyanide-poisoned bac- 

 teria the development of phage T2 is arrested at an early stage 

 (Benzer and Jacob, 1953). Streptomycin added either before 

 or after primary infection prevents superinfection breakdown, 

 even in streptomycin-resistant bacteria in which phage develop- 

 ment is normal. Deoxyribonuclease isolated from E. coli is in- 

 hibited by streptomycin. 



10. Heat-killed phage T2 is inactive both in primary infection 

 and superinfection, whereas ultraviolet-inactivated T2 performs 

 either role in superinfection breakdown. 



7 7. In all experiments recorded above the bacteria were grown 

 in tryptose broth. KozlofF (1952a) records a personal commu- 

 nication from Graham that superinfection breakdown does not 

 occur with bacteria which have been grown in ammonium lac- 

 tate medium. It likewise does not occur when magnesium is de- 

 ficient. Hershey, Garen, Fraser, and Hudis (1954) report that 

 reduction of the magnesium level in their cultures to 10~° M 

 blocks superinfection breakdown, presumably by inhibiting bac- 

 terial deoxyribonuclease. They also state that the limitation of 

 breakdown at about 50 per cent is due to incomplete injection 

 by superinfecting phage. 



