# DNA mismatch repair mediated by Mlh1–Pms1 endonuclease-catalyzed mispair excision

## TL;DR

This study reveals a new DNA repair pathway involving Mlh1-Pms1 that helps fix DNA errors and prevents cancer.

## Contribution

The study identifies a new Mlh1-Pms1 endonuclease-mediated excision pathway in DNA mismatch repair.

## Key findings

- Mlh1-Pms1 endonuclease activity directly catalyzes mispair excision in DNA mismatch repair.
- The Mlh1-Pms1 pathway accounts for most or all DNA mismatch repair when combined with Exo1 and Rad27.
- The pathway forms single-strand DNA gaps of variable sizes during repair.

## Abstract

DNA mismatch repair (MMR) prevents mutations due to DNA replication errors and suppresses the development of cancers. Previously identified mispair excision pathways dependent on either Exonuclease 1 or Rad27 only appear to account for a small fraction of MMR. Here, we identified and reconstituted an Mlh1-Pms1 endonuclease-mediated excision reaction that along with the Exonuclease 1 and Rad27 excision pathways appears to account for most or all of MMR. These results identify an unappreciated role for the Mlh1-Pms1 endonuclease in directly catalyzing mispair excision that provides a comprehensive view of the range of excision pathways that function in MMR and also provides an explanation for the universal requirement for Mlh1-Pms1 in MMR.

Eukaryotic DNA mismatch repair (MMR) involves several excision pathways, including those mediated by exonuclease 1 (Exo1) and by the flap endonuclease Rad27 (human FEN1) coupled with DNA polymerase δ. Simultaneous inactivation of both excision mechanisms causes an MMR defect that is at most 5 to 13% of that caused by complete inactivation of MMR. Here, we reconstituted nicked-strand-specific MMR with the Saccharomyces cerevisiae proteins Msh2-Msh6 or Msh2-Msh3, DNA polymerase ε, RFC, PCNA, RPA, and Mlh1-Pms1 (human Mlh1-Pms2) under conditions lacking Exo1, Rad27, or strand-displacement synthesis by DNA polymerase δ. These reactions required the Mlh1-Pms1 endonuclease activity, its activation by RFC and PCNA, and its recruitment by Msh2-Msh6 or Msh2-Msh3. MMR was mediated by nicked-strand-specific excision by Mlh1-Pms1 through formation of single-strand DNA gaps having a broad range of sizes. This reaction is consistent with genetic data demonstrating redundancy between the Exo1, Rad27, and Mlh1-Pms1 excision pathways in MMR.