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A method was devised for identifying nonlethal mutants of T4 bacteriophage which lack the capacity to induce degradation of the deoxyribonucleic acid (DNA) of their host,Escherichia coli. If a culture is infected in a medium containing hydroxyurea (HU), a compound that blocks de novo deoxyribonucleotide biosynthesis by interacting with ribonucleotide reductase, mutant phage that cannot establish the alternate pathway of deoxyribonucleotide production from bacterial DNA will fail to produce progeny. The progeny of 100 phages that survived heavy mutagenesis with hydroxylamine were tested for their ability to multiply in the presence of HU. Four of the cultures lacked this capacity. Cells infected with one of these mutants, designated T4nd 28, accumulated double-stranded fragments of host DNA with a molecular weight of approximately 2 × 108 daltons. This mutant failed to induce T4 endonuclease II, an enzyme known to produce single-strand breaks in double-stranded cytosine-containing DNA. The properties ofnd 28 give strong support to an earlier suggestion that T4 endonuclease II participates in host DNA degradation. Thend 28 mutation mapped between T4 genes 32 and 63 and was very close to the latter gene. It is, thus, in the region of the T4 map that is occupied by genes for a number of other enzymes, including deoxycytidylate deaminase, thymidylate synthetase, dihydrofolate reductase, and ribonucleotide reductase, that are nonessential to phage production in rich media.

Isolation of Bacteriophage T4 Mutants Defective in the Ability to Degrade Host Deoxyribonucleic Acid