While in Escherichia coli and closely related bacteria MutH protein display the endonuclease activity necessary to direct the postreplicative DNA mismatch repair system (MRS) to the newly synthesized DNA strand, in the rest of bacteria and in eukaryotes, MutL homologues take on this rol. MutH recognizes and nicks a transitory unmethylated GATC site in the newly synthesized DNA strand. However, the endonuclease activity of MutL homologues seems to be independent of any methylation process, and the sequence where they cut the DNA molecule has never been studied.
The aim of this work is to characterize the Pseudomonas aeruginosa MutL (PaMutL) endonuclease target sequence. Using plasmid DNA and different methodologies (second digestion with a restriction enzyme followed by agarose gels electrophoresis and ethidium bromide staining, or denaturing polyacrylamide gels electrophoresis and silver staining, etc.) we analyzed PaMutL target DNA sites.
Our results showed that PaMutL is able to cut plasmid DNA at different sites; however no specific sequence seems to be recognized. This result represents a significant difference in the functioning of the MRS between bacteria strains in which the MRS is directed by MutH or MutL, and could have implications in genomes stability and consequently on adaptative and evolutionary processes.