DNA polymerase theta is a low-fidelity family A enzyme implicated in translesion synthesis and in somatic hypermutation
DNA polymerase theta is a low-fidelity family A enzyme implicated in translesion synthesis (TLS) and in somatic hypermutation (SHM). DNA-dependent DNA polymerases can be classified in six main groups based upon phylogenetic relationships with E. coli polymerase I (classA), E. coli polymerase II (class B), E.coli polymerase III (class C), euryarchaaeota polymerase II (class D), human polymerase beta (class x), E. coli UmuC/DinB and eukaryotic RAP 30/Xeroderma pigmentosum variant (class Y). Family A polymerase functions primarily to fill DNA gaps that arise during DNA repair, recombination and replication. Pol theta is an exception among family A polymerases and generates processive single base substitutions. Family A polymerase are found primarily in organisms related to prokaryotes and include prokaryotic DNA polymerase I (pol I) ,mitochondrial polymerase delta, and several bacteriphage polymerases including those from odd-numbered phage (T3, T5, and T7). Prokaryotic Pol Is have two functional domains located on the same polypeptide; a 5'-3' polymerase and 5'-3' exonuclease. Pol I uses its 5' nuclease activity to remove the ribonucleotide portion of newly synthesized Okazaki fragments and DNA polymerase activity to fill in the resulting gap. Polymerase theta mostly has amino-terminal helicase domain, a carboxy-terminal polymerase domain and an intervening space region.
Comment:The Pol A domain has a shape of a right hand in which the palm, fingers and thumb form the DNA-binding crevice; the active site, composed of three acidic residues, is located at the palm which forms the base of the crevice.