5' -] 3'-EXONUCLEASES OF BACTERIOPHAGE-T4

Two enzyme activites 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 UV-irradiated DNA which was treated with T4 endonuclease V. One of the activities, referred to as T4 exonuclease B, was purified 400-fold from an extract of T4v1-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 Mg2+. The MW of the enzyme, as estimated by gel filtration, is approximatley 35,000. Another activity, referred to as T4 exonuclease C, was purified 240-fold from the extract. This activity also excises pyrimidine dimers from UV-irradiated, incised DNA and releases nucleotides at 5'' termini. It has a pH optimum at 7.5 and requires Mg2+. The MW of the enzyme is approximately 20,000.