SUMMARY

            MutS, MutL and MutH are the essential components of the mismatch repair system in Escherichia coli. MutS recognizes and binds the mismatch, then MutL binds to MutS and the complex activates the d(GATC) endonuclease activity of MutH, which cleaves the newly synthesized, unmethylated strand. Pseudomonas aeruginosa has the mutS and mutL, but not the mutH homologues genes. The deduced amino acids sequence of MutS and MutL from P. aeruginosa shares 58% and 45% identity with the corresponding proteins of E. coli, respectively. We show here that the mutL, but not the mutS gene from P. aeruginosa is able to fully complement the corresponding E. coli mutant, and vice versa. In vitro, MutL from P. aeruginosa is not able to activate the E. coli MutH endonuclease in a mismatch-independent manner, confirming that the E. coli mutant complementation is not due to an unspecific activation of MutH, but a functional complementation. In silico analysis shows that the N-terminal region of the MutL from P. aeruginosa (326 aa), the most conserved region in the MutL family, shares 63% identity with the corresponding region of the MutL protein of E. coli. These results allow to narrow down MutL regions that are important for the interaction with MutS and MutH, and have implications for the evolution, structure and function of bacterial DNA repair enzymes.