The Escherichia coli DNA Mismatch Repair (MMR) protein MutS exist as dimers and 
tetramers in solution, and the identification of its functional oligomeric state 
has been matter of extensive study. In the present work, we have analyzed the 
oligomerization state of MutS from Pseudomonas aeruginosa a bacterial species 
devoid of Dam methylation and MutH homologue. By analyzing native MutS and 
different mutated versions of the protein, we determined that P. aeruginosa MutS 
is mainly tetrameric in solution and that its oligomerization capacity is 
conducted as in E. coli, by the C-terminal region of the protein. The analysis 
of mismatch oligonucleotide binding activity showed that wild-type MutS binds to 
DNA as tetramer. The DNA binding activity decreased when the C-terminal region 
was deleted (MutSDelta798) or when a full-length MutS with tetramerization 
defects (MutSR842E) was tested. The ATPase activity of MutSDelta798 was similar 
to MutSR842E and diminished respect to the wild-type protein. Experiments 
carried out on a P. aeruginosa mutS strain to test the proficiency of different 
oligomeric versions of MutS to function in vivo showed that MutSDelta798 is not 
functional and that full-length dimeric version MutSR842E, is not capable of 
completely restoring the MMR activity of the mutant strain. Additional 
experiments carried out in conditions of high mutation rate induced by the base 
analogue 2-AP confirm that the dimeric version of MutS is not as efficient as 
the tetrameric wild-type protein to prevent mutations. Therefore, it is 
concluded that although dimeric MutS is sufficient for MMR activity, optimal 
activity is obtained with the tetrameric version of the protein and therefore it 
should be considered as the active form of MutS in P. aeruginosa.