Escherichia coli MutS and MutL proteins play a crucial role in the methyl-directed mismatch repair pathway, very-short-patch repair, transcription-coupled nucleotide excision repair, and the prevention of the homeologous recombination; playing an important role in genetic stability. It has been shown that the expression of these proteins in E. coli can be modulated by cell physiology and differentiation, which would let cells to regulate their potential to evolve. Nothing is known about the regulation of the expression of these proteins in P. aeruginosa, a very versatile bacterium. In order to analyze the expression of P. aeruginosa MutS, we generated a mutant strain in which the endogenous mutS gene was replaced by a new copy having 18 extra nucleotides that codify for six histidine residues before the stop codon. In this way the endogenous MutS protein is synthesized having six histidine residues at the C-terminal region which let us to identify it using antibodies directed against the his-tag, or purify and concentrate it using metal chelation chromatography. Using this strain we observed, as described for E. coli, that the level of expression of P. aeruginosa MutS in cultures grown exponentially is higher than in stationary phase. These results suggest that regulation of MMRS proteins could be a common mechanism used by prokaryotes to modulate adaptive processes.