A recJ homolog was cloned from the extremely thermophilic bacterium Thermus themophilus HB8. It encodes a 527 amino acid protein that has 33% identity to Escherichia coliRecJ protein and includes the characteristic motifs conserved among RecJ homologs. Although T.thermophilus RecJ protein (ttRecJ) was expressed as an inclusion body, it was purified in soluble form through denaturation with urea and subsequent refolding steps. Limited proteolysis showed that ttRecJ has a protease-resistant core domain, which includes all the conserved motifs. We constructed a truncated ttRecJ gene that corresponds to the core domain (cd-ttRecJ). cd-ttRecJ was overexpressed in soluble form and purified. ttRecJ and cd-ttRecJ were stable up to 60°C. Size exclusion chromatography indicated that ttRecJ exists in several oligomeric states, whereas cd-ttRecJ is monomeric in solution. Both proteins have 5′→3′ exonuclease activity, which was enhanced by increasing the temperature to 50°C. Mg2+, Mn2+ or Co2+ ions were required to activate both proteins, whereas Ca2+ and Zn2+ had no effects.
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