Family A and family B DNA polymerases are structurally related: evolutionary implications.

In order to establish the evolutionary relationship between the family A and B DNA polymerases, we have closely compared the 3'-->5' exonuclease domains between the Klenow fragment of E.coli DNA polymerase I (a family A DNA polymerase) and the bacteriophage PRD1 DNA polymerase, the smallest member of the DNA polymerase family B. Although the PRD1 DNA polymerase has a smaller 3'-->5' exonuclease domain, its active sites appear to be very similar to those of the Klenow fragment. Site-directed mutagenesis studies revealed that the residues important for the 3'-->5' exonuclease activity, particularly metal binding ligands for the Klenow fragment, are all conserved in the PRD1 DNA polymerase as well. The metal binding ligands are also essential for the strand-displacement activity of the PRD1 DNA polymerase. Based on these results and the studies by others in various systems, we conclude that family A and B DNA polymerases, at least in the 3'-->5' exonuclease domain, are structurally as well as evolutionarily related.