Abstract
Excision of deoxyribose-phosphate residues from enzymatically incised abasic sites in double-stranded DNA is required prior to gap-filling and ligation during DNA base excision-repair, and a candidate deoxyribophosphodiesterase (dRpase) activity has been identified in E.coli. This activity is shown here to be a function of the E.coli RecJ protein, previously described as a 5′ - 3′ single-strand specific DNA exonuclease involved in a recombination pathway and in mismatch repair. Highly purified preparations of dRpase contained 5′-3′ exonuclease activity for single-stranded DNA, and homogeneous RecJ protein purified from an overproducer strain had both 5′ - 3′ exonuclease and dRpase activity. Moreover, E.coll recJ strains were deficient in dRpase activity. The hydrolytic dRpase function of the RecJ protein requires Mg2+; in contrast, the activity of E.coll Fpg protein, that promotes the liberation of 5′ – 3′ Rp residues from DNA by β-eliminatlon, is suppressed by Mg2+. Several other E.coll nucleases, including exonucleases I, III, V, and VII, endonucleases I, III and IV and the 5′ – 3′ exonuclease function of DNA polymerase I, are unable to act as a dRpase. Nevertheless, E.coll fpg recJ double mutants retain capacity to repair abasic sites in DNA, indicating the presence of a back-up excision function.