Influence of oxidized purine processing on strand directionality of mismatch repair

Replicative DNA polymerases are high-fidelity enzymes that misincorporate nucleotides into nascent DNA with a frequency lower than 1/105, and this precision is improved to about 1/107 by their proofreading activity. Because this fidelity is insufficient to replicate most genomes without error, nature evolved postreplicative mismatch repair (MMR), which improves the fidelity of DNA replication by up to three orders of magnitude through correcting biosynthetic errors that escaped proofreading. MMR must be able to recognize non-Watson-Crick base pairs and excise the misincorporated nucleotides from the nascent DNA strand, which carries - by definition - the erroneous genetic information. In eukaryotes, MMR is believed to be directed to the nascent strand by pre-existing discontinuities such as gaps between Okazaki fragments in the lagging strand, or breaks in the leading strand generated by the mismatch-activated endonuclease of the MutL homologs PMS1 in yeast or PMS2 in vertebrates. We recently demonstrated that the eukaryotic MMR machinery can make use also of strand breaks arising during excision of uracils or ribonucleotides from DNA. We now show that intermediates of MYH-dependent excision of adenines mispaired with 8-oxoguanine (GO) also act as MMR initiation sites in extracts of human cells or Xenopus laevis eggs. Unexpectedly, GO/C pairs were not processed in these extracts and failed to affect MMR directionality, but extracts supplemented with exogenous OGG1 did so. Because OGG1-mediated excision of GO might misdirect MMR to the template strand, our findings suggest that OGG1 activity might be inhibited during MMR