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Topoisomerase-primase domain. This is a nucleotidyl transferase/hydrolase domain found in type IA, type IIA and type IIB topoisomerases, bacterial DnaG-type primases, small primase-like proteins from bacteria and archaea, OLD family nucleases from bacterial and archaea, and bacterial DNA repair proteins of the RecR/M family. This domain has two conserved motifs, one of which centers at a conserved glutamate and the other one at two conserved aspartates (DxD). This glutamate and two aspartates, cluster together to form a highly acid surface patch. The conserved glutamate may act as a general base in nucleotide polymerization by primases and in strand joining in topoisomerases and, as a general acid in strand cleavage by topisomerases and nucleases. The DXD motif may co-ordinate Mg2+, a cofactor required for full catalytic function.
Jiyao W et al.(2023). "The conserved domain database in 2023.",