Escherichia coli MutS, MutL and MutH proteins act sequentially in the MMRS (mismatch repair system). MutH directs the repair system to the newly synthesized strand due to its transient lack of Dam (DNA-adenine methylase) methylation. Although Pseudomonas aeruginosa does not have the corresponding E. coli MutH and Dam homologues, and consequently the MMRS seems to work differently, we show that the mutL gene from P. aeruginosa is capable of complementing a MutL-deficient strain of E. coli. MutL from P. aeruginosa has conserved 21 out of the 22 amino acids known to affect functioning of E. coli MutL. We showed, using protein affinity chromatography, that the C-terminal regions of P. aeruginosa and E. coli MutL are capable of specifically interacting with E. coli MutH and retaining the E. coli MutH. Although, the amino acid sequences of the C-terminal regions of these two proteins are only 18% identical, they are 88% identical in the predicted secondary structure. Finally, by analysing (E. coliP. aeruginosa) chimaeric MutL proteins, we show that the N-terminal regions of E. coli and P. aeruginosa MutL proteins function similarly, in vivo and in vitro. These new findings support the hypothesis that a large surface, rather than a single amino acid, constitutes the MutL surface for interaction with MutH, and that the N- and C-terminal regions of MutL are involved in such interactions.

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