SUMMARY

             Escherichia coli MutS, MutL and MutH proteins act sequentially in the mismatch repair system, which repairs DNA replication errors. We have previously shown that mutL from Pseudomonas aeruginosa complements an E. coli MutL-deficient strain, although P. aeruginosa does not have the corresponding E. coli MutH homologue. Here we use the P. aeruginosa MutL protein as a tool to further analyze the interaction MutL-MutH. We compared and analyzed the amino acids sequences and the predicted secondary structures of the E. coli and P. aeruginosa MutL proteins, and analyzed the in vitro and in vivo functioning of the wild type and chimeric MutL proteins (P. aeruginosa/E. coli). We also used protein affinity chromatography to analyze the interactions between the N- and C-terminal regions of P. aeruginosa and E. coli MutL proteins with the E. coli MutH protein. These analysis revealed that the C-terminal region of MutL proteins interact with MutH. Although the C-terminal regions of MutL proteins from P. aeruginosa and E. coli have only 18% of amino acid identity, they have 85% of identity in the predicted secondary structure. These results are in agreement and further support the hypothesis that a large surface of MutL, rather than single amino acids, is involved in the interaction with MutH, and that the C-terminal region of the MutL protein participates in the MutL-MutH interaction, although the N-terminal region is crucial for the stimulation of the MutH endonuclease activity.