A novel Mre11 protein from the hyperthermophilic euryarchaeon Thermococcus barophilus Ch5 possesses 5'-3' exonuclease and endonuclease activities.

Mre11 is one of important proteins that are involved in DNA repair and recombination by processing DNA ends to produce 3'-single stranded DNA, thus providing a platform for other DNA repair and recombination proteins. In this work, we characterized the Mre11 protein from the hyperthermophilic euryarchaeon Thermococcus barophilus Ch5 (Tba-Mre11) biochemically and dissected the roles of its four conserved residues, which is the first report on Mre11 proteins from Thermococcus. Tba-Mre11 possesses exonuclease activity for degrading ssDNA and dsDNA in the 5'-3' direction, which contrasts with other reported Mre11 homologs. Maximum degradation efficiency was observed with Mn at 80 °C and at pH 7.5-9.5. In addition to possessing 5'-3' exonuclease activity, Tba-Mre11 has endonuclease activity that nicks plasmid DNA and circular ssDNA. Mutational data show that residues D10, D51 and N86 in Tba-Mre11 are essential for DNA degradation since almost no activity was observed for the D10A, D51A and N86A mutants. By comparison, residue D44 in Tba-Mre11 is not responsible for DNA degradation since the D44A mutant possessed the similar WT protein activity. Notably, the D44A mutant almost completely abolished the ability to bind DNA, suggesting that residue D44 is essential for binding DNA.