Abstract

The homologues of MutS proteins are widespread among both Prokaryotes and Eukaryotes. MutS designated as MutS1 is a part of MMR (mismatch repair) system which is responsible for removal of mispaired bases and small insertion/deletion loops in DNA. Initially, the only MutS homologues known were those engaged in mismatch repair and these were later designated as MutS1. Subsequently, the MutS2 homologue was distinguished. MutS2 does not recognize mispaired bases but it acts as the inhibitor of RecA- dependent homologous recombination. The phylogenetic analysis of multiple aminoacid sequences of MutS proteins revealed the existence of three other MutS homologues: MutS3, MutS4 and MutS5. The main feature of the three homologues is the lack of both the N-terminal domain typical of MutS1, which is responsible for mismatch recognition, and the C-terminal endonuclease domain, characteristic of MutS2. The analysis of representative prokaryotic genomes showed that the MutS3 homologue in a species is always accompanied by MutS1 and MutS2. The retention of MutS3 together with MutS1 and MutS2 suggests that this homologue gives some evolutionary benefit and that MutS3 may play an important role in DNA metabolism. However, until now MutS3 has not been isolated and studied in terms of its biological function. Staphylococcus aureus is the etiological factor of numerous infections and it is frequently reported to develop antibiotic resistance. As S. aureus pathogenicity is directly connected with the ability for adaptation and genetic plasticity, it is of great concern to clarify whether MutS3 is involved in DNA repair processes. Assuming that MutS3 participates in an unknown mechanisms involved in the control of genetic variability, it could be associated with important medical implications. The gene coding for S. aureus MutS3 was PCR amplified and cloned into a T7 expression vector. The efficient expression of 61 kDa his-tagged MutS3 was obtained in Escherichia coli cells. Unfortunately, the protein formed inclusion bodies and was insoluble in all buffers tested. The computer prediction based on the amino acid sequence revealed the presence of transmembrane regions. The analysis of MutS3 amino acid sequences from S. aureus and other bacteria identified the presence of an ATPase domain typical of MutS homologues but provided no convincing evidence for the presence of a DNA binding domain.

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