Replicative DNA polymerases (EC 2.7.7.7) are the key enzymes catalyzing the accurate replication of DNA. They require either a small RNA molecule or a protein as a primer for the de novo synthesis of a DNA chain. On the basis of sequence similarity, a number of DNA polymerases have been grouped [1,2,3,4,5,6,7] under the designation of DNA polymerase family B. These are:

• Higher eukaryotes polymerases α.
• Higher eukaryotes polymerases delta.
• Yeast polymerase I/α (gene POL1), polymerase II/epsilon (gene POL2), polymerase III/delta (gene POL3) and polymerase REV3.
• Escherichia coli polymerase II (gene dinA or polB).
• Archaebacterial polymerases.
• Polymerases of viruses from the herpesviridae family.
• Polymerases from Adenoviruses.
• Polymerases from Baculoviruses.
• Polymerases from Chlorella viruses.
• Polymerases from Poxviruses.
• Bacteriophage T4 polymerase.
• Podoviridae bacteriophages Phi-29, M2 and PZA polymerase.
• Tectiviridae bacteriophage PRD1 polymerase.
• Polymerases encoded on mitochondrial linear DNA plasmids in various fungi and plants (Kluyveromyces lactis pGKL1 and pGKL2, Agaricus bitorquis pEM, Ascobolus immersus pAI2, Claviceps purpurea pCLK1, Neurospora Kalilo and Maranhar, maize S-1, etc).

Six regions of similarity (numbered from I to VI) are found in all or a subset of the above polymerases. The most conserved region (I) includes a conserved tetrapeptide with two aspartate residues. Its function is not yet known. However, it has been suggested [3] that it may be involved in binding a magnesium ion. We selected this conserved region as a signature for this family of DNA polymerases.