Human correxonuclease. Purification and properties of a DNA repair exonuclease from placenta

An exonuclease, which hydrolyzes single-stranded DNA, has been purified from human placenta. It initiates hydrolysis at both the 3' and 5' termini of such DNA with equal facility, yielding 5'-phosphorylated oligonucleotides averaging 4 nucleotides in length. These oligonucleotides are released from both termini at equal rates with the same size distribution. Although not detectably active against intact native DNA, this enzyme can initiate hydrolysis at single-stranded breaks, creating a gap 30 to 40 nucleotides long. If a pyrimidine dimer is adjacent to this break, the enzyme, by virtue of its action at internal phosphodiester bonds, can excise such dimers. We refer to this exonuclease as human correxonuclease because of its excision capability.