Temporal exclusion and breakdown of superinfecting virus deoxyribonucleic acid were measured after infection with T4 and T5 bacteriophages of normal strains of and strains deficient in endonuclease-I. Bacteria deficient in endonuclease-I when infected with T4 phage excluded superinfecting T4 with little solubilization of the secondary DNA. With wild-type bacteria exclusion was accompanied by extensive superinfection breakdown, probably caused by the bacterial endonuclease-I. In bacteria infected by T5 phage, superinfecting T2 phages could be excluded even when deoxyribonucleic acid degradation was inhibited by maintaining a low [Mg] in the growth medium. In the presence of 0.01 -magnesium ions, both wild-type bacteria and bacteria deficient in endonuclease-I infected with T5 phage produced extensive solubilization of the DNA of superinfecting T2 or T4 phages. A nuclease induced by T5 was probably partly responsible for the DNA breakdown which occurred in these conditions.


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