Mismatch repair proteins collaborate with methyltransferases in the repair of O6-methylguanine

DNA repair is essential for combatting the adverse effects of damage to the genome. One example of base damage is O6-methylguanine (O6mG), which stably pairs with thymine during replication and thereby creates a promutagenic O6mG:T mismatch. This mismatch has also been linked with cellular toxicity. Therefore, in the absence of repair, O6mG:T mismatches can lead to cell death or result in G:C → A:T transition mutations upon the next round of replication. Cysteine thiolate residues on the Ada and Ogt methyltransferase (MTase) proteins directly reverse the O6mG base damage to yield guanine. When a cytosine is opposite the lesion, MTase repair restores a normal G:C pairing. However, if replication past the lesion has produced an O6mG:T mismatch, MTase conversion to a G:T mispair must still undergo correction to avoid mutation. Two mismatch repair pathways in E. coli that convert G:T mispairs to native G:C pairings are methyl-directed mismatch repair (MMR) and very short patch repair (VSPR). This work examined the possible roles that proteins in these pathways play in coordination with the canonical MTase repair of O6mG:T mismatches. The possibility of this repair network was analyzed by probing the efficiency of MTase repair of a single O6mG residue in cells deficient in individual mismatch repair proteins (Dam, MutH, MutS, MutL, or Vsr). We found that MTase repair in cells deficient in Dam or MutH showed wild-type levels of MTase repair. In contrast, cells lacking any of the VSPR proteins MutS, MutL, or Vsr showed a decrease in repair of O6mG by the Ada and Ogt MTases. Evidence is presented that the VSPR pathway positively influences MTase repair of O6mG:T mismatches, and assists the efficiency of restoring these mismatches to native G:C base pairs.