| Type: | Family | Name: | DNA polymerase family X |
| Description: | DNA carries the biological information that instructs cells how to exist in an ordered fashion: accurate replication is thus one of the mostimportant events in the cell life cycle. This function is mediated by DNA-directed DNA-polymerases, which add nucleotide triphosphate (dNTP)residues to the 5'-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 beta-strand and an alpha-helix; motif B contains an alpha-helix with positive charges; and motif C has a doublet of negative charges, located in a beta-turn-beta secondary structure [].DNA polymerases () can be classified, on the basis of sequence similarity [, ], into at least four different groups: A, B, C and X. Members of family X are small (about 40 kDa) 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, while the latter terminally adds single nucleotides to polydeoxynucleotide chains.Both enzymes catalyse addition of nucleotides in a distributive manner, i.e. they dissociate from the template-primer after addition of each nucleotide.DNA-polymerases show a degree of structural similarity with RNA-polymerases. | Short Name: | DNA_pol_X |
