Genomic analysis of a hyperthermophilic archaeon, Thermococcus sp. NA1, revealed an ORF of 1689 bases encoding 562 amino acids that showed a high similarity to DNA ligases from other hyperthermophilic archaea. The ligase, which was designated TNA1_ lig ( Thermococcus sp. NA1 ligase), was cloned and expressed in Escherichia coli. The recombinant TNA1_ lig was purified by metal affinity chromatography. The optimum ligase activity of the recombinant TNA1_ lig occurred at 80 degrees C and pH 7.5. The enzyme was activated by MgCl2 and ZnCl2 but was inhibited by MnCl2 and NiCl2. Additionally, the enzyme was activated by either ATP or NAD(+).