Errors introduced during replication can be corrected by the mismatch repair system found in most organisms. A mismatch is recognized by MutS, which then recruits MutL and the endonuclease MutH. MutH nicks the non-methylated DNA at a hemi-methylated GATC site, providing the signal to distinguish the parental strand from the newly synthesized strand. Most genomes contain MutS and MutL homologs, but only few contain a MutH homologous. Therefore, most organisms must use a different nicking endonuclease. It was recently described that human MutLa is a latent endonuclease that is activated in a mismatch-dependent manner. The probable endonuclease active site has been localized to a DQHA(X)2E(X)4E motif, which is conserved in eukaryotic homologs and in MutL proteins from a number of bacterial species but is lacking in MutL proteins from bacteria that have a MutH homologous. Pseudomonas aeruginosa does not contain a MutH homologous, and has only partially conserved the MutL endonuclease motif. Moreover, we have recently showed that Escherichia coli MutL protein complements a MutL deficient strain of P. aeruginosa. These results suggest that MutL endonuclease in bacteria could be part of the MMRS in some bacterial species that do not have a MutH homologous, but not necessarily in all of them. The objective of the present project is to analyze MutL proteins and MutL deficient strains from different bacteria species, and determine the putative endonucleolytic function of MutL in bacterial mismatch repair