ИСТИНА

The DNA mismatch repair (MMR) pathway removes errors that appear during genome replication. MMR corrects DNA by excising an extended single­stranded fragment of the newly synthesized DNA and then filling the resulting gap. The signal for the excision is hydrolysis of the phosphodiester bond in one of the DNA strands. In γ­proteobacteria this break is introduced by the MutH. However, most prokaryotes and all eukaryotes lack a mutH gene. Presumably, in these organisms MutL homologs cut the nascent strand of the DNA. Conserved endonuclease motifs were identified in these MutL. In the alignment of 1792 sequences of bacterial MutL homologs we identified five endonuclease motifs comprising the catalytic site responsible for DNA cleavage in 1390 sequences. MutL proteins possessing the endonuclease motifs have been purified only from five bacterial species. Mechanism of searching of cleavage site in DNA by MutL is still unclear. We have purified the MutL protein from Rhodobacter sphaeroides (rsMutL) for the first time. We demonstrated the influence of metal ions on specificity and efficiency of DNA hydrolysis by rsMutL. The protein showed the highest activity in the presence of Mn2+. The extent of plasmid DNA hydrolysis declined in the row Mn2+>Co2+>Mg2+>Cd2+; Ni2+ and Ca2+ did not activate rsMutL. Zn2+ inhibited rsMutL endonuclease activity in thepresence of Mn2+ excess. Apparently, Zn2+ is not involved in DNA cleavage by rsMutL and could perform a structural function. Mn2+and Mg2+ together induced double strand break followed by an increase in the amount of linear DNA form that could be a result of rsMutL second subunit activation. Analysis of amino acid sequences and biochemical properties of studied bacterial MutL homologs with endonuclease activity revealed that rsMutL is similar the MutL proteins from N.gonorrhoeae and P. aeruginosa.