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 similarities a number of DNA polymerases have been grouped together [1,2,3] under the designation of DNA polymerase family A. The polymerases that belong to this family are listed below.

• Escherichia coli and various other bacterial polymerase I (gene polA).
• Thermus aquaticus Taq polymerase.
• Bacteriophage sp01 polymerase.
• Bacteriophage sp02 polymerase.
• Bacteriophage T5 polymerase.
• Bacteriophage T7 polymerase.
• Mycobacteriophage L5 polymerase.
• Yeast mitochondrial polymerase γ (gene MIP1).

Five regions of similarity are found in all the above polymerases. One of these conserved regions, known as 'motif B' [1], is located in a domain which, in Escherichia coli polA, has been shown to bind deoxynucleotide triphosphate substrates; it contains a conserved tyrosine which has been shown, by photo-affinity labelling, to be in the active site; a conserved lysine, also part of this motif, can be chemically labelled, using pyridoxal phosphate. We use this conserved region as a signature for this family of DNA polymerases.