DNA polymerase family

On the basis of their sequence homology, DNA polymerases can be divided into seven different families.

Family A
Family A polymerases contain both replicative (e.g.T7 DNA polymerase,eukaryotic mitochondrial DNA Polymerase γ) and repair polymerases (E. coli DNA pol I, Thermus aquaticus pol I, and Bacillus stearothermophilus pol I).


Family B
Family B polymerases mostly contain major eukaryotic replicative polymerases like α, δ, ε, and also DNA polymerase ζ. It also includes DNA polymerases encoded by some bacteria and bacteriophages like from T4, Phi29 and RB69 bacteriophages. These enzymes are involved in both leading and lagging strand synthesis with high accuracy. A hallmark of the B family of polymerases is remarkable accuracy Except DNA polymerase α and ζ, all member of this family have proofreading activity.

Family C
Member of this family are the primary bacterial chromosomal replicative enzymes with 3’-5' exonuclease activity.

Family D
These polymerases are still not very well characterized. All known members are found in the Euryarchaeota subdomain of Archaea and are thought to be replicative polymerases.

Family X
Family X eukaryotic polymerase β, σ,λ,μ, and terminal deoxynucleotidyl transferase (TdT). Pol β is required for short-patch base excision repair (BER). Pol λ and Pol μ are involved in non-homologous end joining (NHEJ). TdT is only expressed in lymphoid tissue and adds "n nucleotides" to double-strand breaks formed during V(D)J recombination to promote immunological diversity. The yeast Saccharomyces cerevisiae has only one Pol X polymerase, Pol4, which is involved in NHEJ. Genome analysis of plants shows that gene for polymerase β and TdT is not present in plants.

Family Y
These polymerases have low fidelity on undamaged templates and r ability to replicate through damaged DNA. So they are also called translesion sythesis (TLS) polymerases. Depending on the lesion TLS polymerases can bypass the damage in an error-free or error-prone fashion. Members in humans are Pol ι (iota), Pol κ (kappa) and Rev1 (terminal deoxycytidyl transferase). In E.coli two TLS polymerases, Pol IV (DINB) and PolV (UMUC), are reported.

Family RT
The reverse transcriptase family contains members both from retroviruses and eukaryotic polymerases. The eukaryotic polymerases are usually restricted to telomerases. These polymerases use a RNA template to synthesize the DNA strand.HIV is a Retro virus.