SUMMARY
In Escherichia coli, DNA mismatch repair (MMR) is initiated by MutS binding to a mismatch followed by MutL binding. MutL activates the endonuclease MutH which cleaves the unmethylated daughter strand at GATC sites. We showed that Pseudomonas aeruginosa mutL gene restore approximately 80% the MMR system of an E. coli mutL mutant strain. This is an interesting result considering that mutS and mutL, but not mutH homologues have been found in the genome of P. aeruginosa. The first 327 residues of P. aeruginosa and E. coli MutL proteins have 63% of sequence homology and 87% of homology in the predicted secondary structure. In this region, 22 of the 23 amino acids found to be important for the function of MutL in E. coli are present in the P. aeruginosa MutL protein. By contrast, the 300 C-terminal residues have only 18% of sequence homology, but 85% of homology in the predicted secondary structure. MutL chimeric proteins containing the E. coli N-terminal region and the P. aeruginosa C-terminal region or vice versa, also complement (~80%) the MMR system of an E. coli mutL mutant strain. We are analyzing in vitro, the capacity of the wild type and chimeric MutL proteins to interact with and to stimulate the MutH endonuclease activity. These results support the hypothesis that the N-terminal region of MutL contains most of the functional activities for the MMR system, while the C-terminal region is involved principally in the dimerization of the protein.
