E. 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 organism. In order analyze the level of expression of P. aeruginosa MutS, we generated a mutant strain in which the endogenous mutS gene was replaced by a new copy having an extra 18 nucleotides that codify for six histidine residues before the stop codon. In this way the endogenous MutS protein is synthesized with six extra histidine residues at the C-terminal region which let us to purify and concentrate it using metal chelation chromatography. Using this strain we observed that level of expression of P. aeruginosa MutS protein in cultures grown exponentially is considerably higher than in stationary phase, similar to what happen in E. coli, suggesting that regulation of MMRS proteins would be a general way to regulate the potential to evolve.
