SUMMARYTemporal exclusion and breakdown of superinfecting virus deoxyribonucleic acid were measured after infection with T 4 and T 5 bacteriophages of normal strains of Escherichia coli 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 o.oI M-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 T 4 phages. A nuclease induced by T5 was probably partly responsible for the DNA breakdown which occurred in these conditions.

SUMMARYBacteria infected with bacteriophage T 5 were disrupted with lysozyme and mild detergents and the intracellular phage-specific components resolved by sedimentation through neutral sucrose gradients. In pulse-chase experiments with [aH]-thymidine most of the radioactivity initially appeared in a fast-sedimenting form of DNA (fsf) which was very shear-sensitive and bound tightly to nitrocellulose. Label next appeared in a slower sedimenting form (ssf), then phage heads and finally virus particles. The ssf showed susceptibility to shear similar to that of DNA from intact virus, and sedimented with it on neutral gradients. The ssf DNA differed from virus DNA by binding to nitrocellulose and showing a different sedimentation profile on alkaline-sucrose gradients. The pulse-labelled replicating DNA was very heterogeneous in tool. wt. and appeared to contain many single-strand nicks which were extensively repaired while the DNA was still in the fast-sedimenting form. The conversion sequence fsf--+ ssf ---+ heads ---~ phage was supported by the accumulation of components in non-permissive host bacteria infected with certain amber mutants of T 5. One of these, T 5. B1, could not form the T 5 phageinduced 5' exonuclease and in these infections there was no conversion of replicating DNA to ssf, and net DNA synthesis stopped prematurely. It was concluded that maturation of T5 virus involved mature virus-size pieces of DNA of unusual structure as intermediates between replicating DNA and phage heads. The process appeared to require functional T 5 induced exonuclease, but the role of this enzyme was unclear.

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