Mismatch Repair System (MMRS) contributes to genetic stability by correcting point mutations or small insertion/deletion loops and preventing recombination between partially homologous DNA sequences. In Escherichia coli, the assembly of the MMRS is initiated by MutS, an 853 amino acids protein that recognizes and binds to mispaired nucleotides. In vitro, MutS exists as dimers and tetramers in equilibrium. Deletion of the 53 C-terminal amino acids shifts the equilibrium to the dimer form and avoids the formation of inactive high order oligomers. Recently it has been found that this C-terminal region is important for MutS function in vivo suggesting that tetramerization would be critical for the protein activity. In order to determine the role of the C-terminal region on the protein oligomerization and function, several C-terminal deletion mutants of E. coli MutS were constructed. We characterized their oligomerization capacity by light scattering and density gradient sedimentation, and determined their biochemical activity. It was found that the deletion of just 7 amino acids (MutS848 mutant) resulted in the loss of tetramerization and high order aggregate formation. However, MutS848 retained its ATPase activity and DNA binding capability. All C-terminal deletion versions were able to partially complement an E. coli mutS deficient strain suggesting that they are very likely competent for mismatch repair.
