Various plasmids harboring the truncated DNA polymerase gene (polA) from Thermus thermophilus HB8 (Tth polymerase) were constructed. The most thermostable Tth Delta NF2 polymerase [the gene product of polA Delta NF2, which lacked a 751-bp region (region flanked by initiation codon and FspI site in the polA gene)] was selected, and purified from the recombinant Escherichia coli. SDS polyacrylamide gel electrophoresis revealed that the molecular weight of the Tth Delta NF2 polymerase is 58-61 kDa, which is approximately 30 kDa smaller than that of the wild-type enzyme. The specific activity of the 5’to-3’ polymerization of the Tth Delta NF2 polymerase was 63% of that of the Tth polymerase. However, no 5’-to-3’ exonuclease activity was detected in this mutant enzyme (less than 1% of the specific activity of wild-type enzyme). The activities of the wild-type and mutant enzymes were maximal at 75 degrees C. Approximately 50% of the enzyme activity was retained even after heat treatment of the Tth Delta NF2 polymerase at 70 degrees C for 2 h, but the thermostability of the mutant enzyme was slightly lower than that of the wild-type enzyme. Both the Tth Delta NF2 and Tth polymerases were capable of non-templated addition of deoxyribonucleotide to a 3’-hydroxyl group of blunt-ended DNA.