Msh2-Msh3 interferes with DNA metabolism in vivo

Mismatch repair (MMR) is a highly conserved DNA repair pathway that safeguards the genome from errors in DNA replication. In Saccharomyces cerevisiae , two MutS homolog (Msh) complexes, Msh2-Msh3 or Msh2-Msh6, initiate MMR. Msh2-Msh3, the focus of this study, recognizes and directs repair of insertion/deletion loops (IDLs) up to ~17 nucleotides. Msh2-Msh3 also recognizes and binds distinct looped and branched DNA structures with varying affinities, thereby contributing to genome stability outside post-replicative MMR through homologous recombination, double-strand break repair (DSBR), and the DNA damage response. Msh2-Msh3 also promotes genome instability through trinucleotide repeat (TNR) expansions. This non-canonical activity is likely an unfortunate consequence of Msh2-Msh3’s intrinsic ability to bind a wide range of DNA structures, including those formed with single-stranded (ss) TNR sequences. We previously demonstrated that Msh2-Msh3 binding to 5’ ssDNA flap structures interfered with the in vitro binding and cleavage activities of the flap endonuclease Rad27 (Fen1 in mammals), which promotes 5’ ssDNA flap processing during Okazaki fragment maturation (OFM) and long-patch base excision repair (LP-BER). Here we demonstrate that elevated Msh2-Msh3 levels interfere with DNA replication and LP-BER in vivo , consistent with the hypothesis that protein abundance and Msh3 ATPase activities are key drivers of Msh2-Msh3-mediated genomic instability. ### Competing Interest Statement The authors have declared no competing interest.