EFFICIENT REPAIR OF ABASIC SITES IN DNA BY MITOCHONDRIAL-ENZYMES

Mutations in mitochondrial DNA (mtDNA) cause a variety of relatively r are human diseases and may contribute to the pathogenesis of other, mo re common degenerative diseases. This stimulates interest in the capac ity of mitochondria to repair damage to mtDNA. Several recent studies have shown that some types of damage to mtDNA may be repaired, particu larly if the lesions can be processed through a base excision mechanis m that employs an abasic site as a common intermediate. In this paper, we demonstrate that a combination of enzymes purified from Xenopus la evis mitochondria efficiently repairs abasic sites in DNA. This repair pathway employs a mitochondrial class II apurinic/apyrimidinic (AP) e ndonuclease to cleave the DNA backbone on the 5' side of an abasic sit e. A deoxyribophosphodiesterase acts to remove the 5' sugar-phosphate residue left by AP endonuclease. mtDNA polymerase gamma fills the resu lting 1-nucleotide gap. The remaining nick is sealed by an mtDNA ligas e. We report the first extensive purification of mtDNA ligase as a 100 -kDa enzyme that functions with an enzyme-adenylate intermediate and i s capable of ligating oligo(dT) strands annealed to poly(rA). These pr operties together with preliminary immunological evidence suggest that mtDNA may be related to nuclear DNA ligase III.