The homologues of MutS, mismatch repair protein, exist in all prokaryotes, with the exception of Actinobacteria, Mollicutes and part of the Archaea. Multiple alignments of 316 MutS amino acid sequences from 169 species revealed conserved residues and sequence motifs distinguishing MutS homologues. All MutS homologues show high conservation within the ATPase domain. MutS1, the homologue responsible for DNA mismatch recognition, is common in Archaea and Bacteria. MutS1 is distinguished by the N-terminal mismatch binding domain containing the GXFXE motif shared by all MutS1 homologues and MSH6 homologues in eukaryotes. Less common than MutS1, MutS2, the suppressor of homologous recombination, is rendered distinctive by the C-terminal Smr endonuclease domain containing the conserved HGXG motif. MutS1 and MutS2 are of enormous significance in maintaining genome integrity. The functions of the other homologues: MutS2-like, MutS3, MutS4, and MutS5 have not yet been found. Each of these homologues exists in a narrower range of taxonomic groups than MutS1 or MutS2 and has neither the mismatch binding nor the Smr domain. The number of different MutS homologues in a single organism usually ranges from one to four; there are rarely five and six only occur exceptionally. The diversity of MutS types and structures begs the question as to how this diversity influenced the evolution of genomes.
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