SnapShot: DNA Mismatch Repair

Mismatch Repair in Bacteria and Eukaryotes Mismatch repair in the bacterium Escherichia coli is initiated when a homodimer of MutS binds as an asymmetric clamp to DNA containing a variety of base-base and insertion-deletion mismatches. The MutL homodimer then couples MutS recognition to the signal that distinguishes between the template and nascent DNA strands. In E. coli, the lack of adenine methylation, catalyzed by the DNA adenine methyltransferase (Dam) in newly synthesized GATC sequences, allows E. coli MutH to cleave the nascent strand. The resulting nick is used for mismatch removal involving the UvrD helicase, single-strand DNA-binding protein (SSB), and excision by single-stranded DNA exonucleases from either direction, depending upon the polarity of the nick relative to the mismatch. DNA polymerase III correctly resynthesizes DNA and ligase completes repair. In bacteria lacking Dam/MutH, as in eukaryotes, the signal for strand discrimination is uncertain but may be the DNA ends associated with replication forks. In these bacteria, MutL harbors a nick-dependent endonuclease that creates a nick that can be used for mismatch excision. Eukaryotic mismatch repair is similar, although it involves several different MutS and MutL homologs: MutSα (MSH2/MSH6) recognizes single base-base mismatches and 1–2 base insertion/deletions; MutSβ (MSH2/MSH3) recognizes insertion/ deletion mismatches containing two or more extra bases. There are three eukaryotic MutL heterodimers: MutLα (in humans MLH1/PMS2; in yeast MLH1/PMS1), MutLβ (MLH1/ MLH3), and MutLγ (human MLH1/PMS1; yeast MLH1/MLH2). The eukaryotic MutS and MutL heterodimers have partial overlap in substrate specificity. MutLα and MutLβ have endonuclease activity, with the active sites present in human PMS2 (yeast PMS1) and human MLH3. The resulting nick can be used for excision by the double-strand DNA 5′-exonuclease activity of Exo1. No helicase has yet been implicated in eukaryotic mismatch repair. Other exonucleases may perform excision, or the mismatch may be removed by strand displacement synthesis. DNA is resynthesized by DNA polymerase δ. Protein Structures Also depicted are structures of proteins involved in DNA mismatch repair (including Protein Data Bank ID, resolution, and species) among the many that are now available. Future studies will likely add to this list, possibly including proteins involved in excision, termination, and coordination of mismatch repair with nucleosome reloading and chromatin remodeling after replication. Evolutionary Conservation The table provides information on the evolutionary conservation of mismatch repair proteins. Additional notes: grey icons indicate genes found only in subset of species; MSH1 in fungi is involved in mitochondrial DNA repair …