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' in equilibrium 3' exonucleases, which are also induced by T4, act at the break to excise dimer-containing nucleotides. We isolated temperature-dependent v mutants, which exhibit increased sensitivity to UV at 42 degrees C but not at 30 degrees C, and found that the mutants induce temperature-sensitive T4 endonuclease V, indicating that the v 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.

T4 endonuclease involved in repair of DNA

An enzyme activity, named T4 endonuclease V, was purified from T4-infected Escherichia coli. The enzyme induces single-stranded breaks in ultraviolet-irradiated DNA but does not act on native or heat-denatured DNA. The enzyme activity is dependent on the dose of ultraviolet irradiation, and the number of the breaks formed is approximately equal to the number of pyrimidine dimers present in the DNA. Denatured DNA, which has been exposed to ultraviolet light, is also attacked by the enzyme although the extent of the reaction is greater with irradiated native DNA. The enzyme shows optimal activity at pH 7.5 and does not require added divalent ions. When the enzyme-treated, irradiated DNA is subjected to stepwise degradation by spleen phosphodiesterase, dimers are released more rapidly than thymine into the acid-soluble fraction, suggesting that the enzyme induces a break at the 5'-side of a pyrimidine dimer. The enzyme, whose formation is controlled by the v(+) gene of T4, appears to be responsible for the first step of excision repair.