DNA polymerase from Sulfolobus solfataricus, strain MT4 (Sso DNA pol), was one of the first archaeal DNA polymerases to be isolated and characterized. Its encoding gene was cloned and sequenced, indicating that Sso DNA pol belongs to family B of DNA polymerases. By limited proteolysis experiments carried out on the recombinant homogeneous protein, we were able to demonstrate that the enzyme has a modular organization of its associated catalytic functions (DNA polymerase and 3'-5' exonuclease). Indeed, the synthetic function was ascribed to the enzyme C-terminal portion, whereas the N-terminal half was found to be responsible for the exonucleolytic activity. In addition, partial proteolysis studies were utilized to map conformational changes on DNA binding by comparing the cleavage map in the absence or presence of nucleic acid ligands. This analysis allowed us to identify two segments of the Sso DNA pol amino acid chain affected by structural modifications following nucleic acid binding: region 1 and region 2, in the middle and at the C-terminal end of the protein chain, respectively. Site-directed mutagenesis studies will be performed to better investigate the role of these two protein segments in DNA substrate interaction.