The gene encoding Sulfophobococcus zilligii DNA polymerase (Szi DNA polymerase) was cloned and sequenced. The Szi DNA polymerase gene encompasses 2394 bp, and codes for a protein consisting of 797 amino acid residues. The deduced amino acid sequence of Szi DNA polymerase exhibited a high degree of similarity with archaeal family B-type DNA polymerase homologues found in both crenarchaeotes and euryarchaeotes (Group I), and contained all of the motifs conserved in the homologues for 3' -> 5' exonuclease and polymerase activities. The Szi DNA polymerase gene was expressed under the control of the T7lac promoter on the expression vector pET-28a(+) in Escherichict coli BL21-CodonPlus(DE3)-RIL. The expressed enzyme was purified by heat treatment and Cibacron blue 3GA column chromatography. The optimum pH of the purified enzyme was determined to be in the range of 6.5-7.0. The enzyme was activated by the magnesium ion, and its activity was inhibited by EDTA and monovalent cations, such as potassium ion and ammonium ion. The half-life of the enzyme at 95 degrees C was calculated to be 4.5 h. Szi DNA polymerase possessed associated 3' -> 5' and 5' -> 3' exonuclease activities. (c) 2005 Elsevier Inc. All rights reserved.