5 ’ + 3 ’ - Exonucleases of Bacteriophage

Two enzyme activities which release nucleotides preferentially from the 5’ termini of DNA were found in T4-infected Escherichia coli. Since no corresponding activities were found in uninfected cells, the activities appear to be induced by T4. Both activities are capable of excising pyrimidine dimers from ultraviolet-irradiated DNA which has been treated with T4 endonuclease V. One of the activities, referred to as T4 exonuclease B, was purified 400-fold from an extract of T4u,infected cells. The enzyme initiates hydrolysis of DNA specifically at the 5’ termini to yield products which are mainly oligonucleotides of varying length. The hydrolysis reaction proceeds in a limited manner. The enzyme shows optimal activity at pH 7.0 and absolutely requires Mg*+. The molecular weight of the enzyme, as estimated by gel filtration, is approximately 35,000. Another activity, referred to as T4 exonuclease C, was purified 240-fold from the extract. This activity also excises pyrimidine dimers from ultraviolet-irradiated, incised DNA and releases nucleotides at 5’ termini. It has a pH optimum at 7.5 and requires Mg 2+. The molecular weight of the enzyme is approximately 20,000.