Sequence analysis of three family B DNA polymerases from the thermoacidophilic crenarchaeon Sulfurisphaera ohwakuensis

Three family B DNA polymerase genes, designated B1, B2, and B3, were cloned from the thermoacidophilic crenarchaeon Sulfurisphaera ohwakuensis, and se quenced. Deduced amino acid sequences of B1 and B3 DNA polymerases have all exonuclease and polymerase motifs which include critical residues for cata lytic activities. Furthermore, a YxGG/A motif, which is located between 3'- 5' exonuclease and polymerization domains of family B DNA polymerases, was also found in each of the B1 and B3 sequences. These findings suggested tha t S. ohwakuensis B1 and B3 DNA polymerases have both exonuclease and polyme rase activities. However, amino acid sequence of the B2 DNA polymerase of t his organism contains several amino acid substitutions in Pol-motifs, and a lso lacks Exo-motif I and Exo-motif II. These substitutions and lack of cer tain motifs raise questions about polymerase and exonuclease activities of the corresponding gene product. The B3 sequence of S. ohwakuensis is more c losely related to Pyrodictium, Aeropyrum, and Archaeoglobus DNA polymerase B3 sequences than to the Sulfolobus B3 sequences. Phylogenetic analysis sho wed that crenarchaeal B1 DNA polymerases are closely related to each other, and suggested that crenarchaeal B3, euryarchaeal family B, and eukaryal ep silon DNA polymerases may be orthologs.