| Primary Identifier | IPR022312 | Type | Family |
| Short Name | DNA_pol_X |
| description | Family X DNA polymerases (PolX) are involved in DNA repair, being evolutionarily conserved in prokaryotes, eukaryotes and archaea. All DNA polymerases from this family are single-subunit enzymes, lacking the 3'-5' exonuclease activity and displaying very low processivity during primer extension reactions []. Proteins in this family include in the well-characterized mammalian Pol beta; more recently discovered eukaryotic polymerases lambda, and mu; and a template-independent polymerase, terminal transferase (TdT) []. In eukaryotes, Pol beta fills short nucleotide gaps produced during base excision repair (BER) []. Pols beta, lamda, mu can also take part in translesion DNA synthesis (TLS) []. Their structures have been revealed [, , ].DNA carries the biological information that instructs cells how to existin an ordered fashion: accurate replication is thus one of the mostimportant events in the cell life cycle. This function is mediated byDNA-directed DNA-polymerases, which add nucleotide triphosphate (dNTP)residues to the 3'-end of the growing DNA chain, using a complementary DNA as template. Small RNA molecules are generally used as primers forchain elongation, although terminal proteins may also be used. Three motifs, A, B and C [], are seen to be conserved across all DNA-polymerases, with motifs A and C also seen in RNA- polymerases. They are centred on invariant residues, and their structural significance was implied from the Klenow (Escherichia coli) structure: motif A contains a strictly-conserved aspartate at the junction of a β-strand and an α-helix; motif B contains an α-helix with positive charges; and motif C has a doublet of negative charges, located in a β-turn-beta secondary structure [].DNA polymerases () can be classified, on the basis of sequencesimilarity [, ], into at least four different groups: A, B, C and X. Members of family X are small (about 40kDa) compared with other polymerases and encompass two distinct polymerase enzymes that have similar functionality: vertebrate polymerase beta (same as yeast pol 4), and terminal deoxynucleotidyl-transferase (TdT) (). The former functions in DNA repair, whilethe latter terminally adds single nucleotides to polydeoxynucleotide chains.Both enzymes catalyse addition of nucleotides in a distributive manner, i.e. theydissociate from the template-primer after addition of each nucleotide.DNA-polymerases show a degree of structural similarity with RNA-polymerases. |
