Enzymic Mechanism of Excision-Repair in T4-Infected Cells

Excision of pyrimidine dimers from ultraviolet-irradiated DNA in a cell-free system of Escherichia coli infected with bacteriophage T4 consists of two different steps, one to induce a single-strand break at a point close to a pyrimidine dimer and the other to release dimer-containing nucleotide from the DNA. The enzymes responsible for these steps were isolated and the reactions were characterized; T4 endonuclease V introduces a break at the 5′ side of a dimer and 5′→3′ exonucleases, which are also induced by T4, act at the break to excise dimer-containing nucleotides. We isolated temperature-dependent ν mutants, which exhibit increased sensitivity to UV at 42°C but not at 30°C, and found that the mutants induce temperature-sensitive T4 endonuclease V, indicating that the y gene of T4 is indeed the structural gene for T4 endonuclease V and that the enzyme is responsible for the first step of excision-repair. A possible mechanism of excision-repair in T4-infected cells is discussed.