Based on sequence homology, DNA polymerases can be further subdivided into seven different families: A, B, C, D, X, Y, and RT.
Polymerases contain both replicative and repair polymerases. Replicative members from this family include the extensively-studied T7 DNA polymerase, as well as the eukaryotic mitochondrial DNA Polymerase γ. Among the repair polymerases are ''E. coli'' DNA pol I, ''Thermus aquaticus'' pol I, and ''Bacillus stearothermophilus'' pol I. These repair polymerases are involved in excision repair and processing of Okazaki fragments generated during lagging strand synthesis.
Polymerases mostly contain replicative polymerases and include the major eukaryotic DNA polymerases α, δ, ε, (see Greek letters) and also DNA polymerase ζ. Family B also includes DNA polymerases encoded by some bacteria and bacteriophages, of which the best-characterized are from T4, Phi29, and RB69 bacteriophages. These enzymes are involved in both leading and lagging strand synthesis during replication. A hallmark of the B family of polymerases is their highly faithful DNA synthesis during replication. While many have an intrinsic 3'-5' proofreading exonuclease activity, eukaryotic DNA polymerases α and ζ are two examples of B family polymerases lacking this proofreading activity.
Polymerases are the primary bacterial chromosomal replicative enzymes. DNA Polymerase III alpha subunit from ''E. coli'' is the catalytic subunit and possesses no known nuclease activity. A separate subunit, the epsilon subunit, possesses the 3'-5' exonuclease activity used for editing during chromosomal replication. Recent research has classified Family C polymerases as a subcategory of Family X.
Polymerases are still not very well characterized. All known examples are found in the Euryarchaeota subdomain of Archaea and are thought to be replicative polymerases.
Contains the well-known eukaryotic polymerase pol β, as well as other eukaryotic polymerases such as pol σ, pol λ, pol μ, and terminal deoxynucleotidyl transferase (TdT). Pol β is required for short-patch base excision repair, a DNA repair pathway that is essential for repairing abasic sites. Pol λ and Pol μ are involved in non-homologous end-joining, a mechanism for rejoining DNA double-strand breaks. TdT is expressed only in lymphoid tissue, and adds "n nucleotides" to double-strand breaks formed during V to promote immunological diversity. The yeast ''Saccharomyces cerevisiae'' has only one Pol X polymerase, Pol4, which is involved in non-homologous end-joining.
Polymerases differ from others in having a low fidelity on undamaged templates and in their ability to replicate through damaged DNA. Members of this family are hence called translesion synthesis (TLS) polymerases. Depending on the lesion, TLS polymerases can bypass the damage in an error-free or error-prone fashion, the latter resulting in elevated mutagenesis. Xeroderma pigmentosum variant (XPV) patients for instance have mutations in the gene encoding Pol η (eta), which is error-free for UV-lesions. In XPV patients, alternative error-prone polymerases, e.g., Polζ (zeta) (polymerase ζ is a B Family polymerase a complex of the catalytic subunit REV3L with Rev7, which associates with Rev1), are thought to be involved in mistakes that result in the cancer predisposition of these patients. Other 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 (UmuD'2C), are known.
The reverse transcriptase family contains examples from both retroviruses and eukaryotic polymerases. The eukaryotic polymerases are usually restricted to telomerases. These polymerases use an RNA template to synthesize the DNA strand.
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