TOPRIM_primases: The topoisomerase-primase (TOPRIM) nucleotidyl transferase/hydrolase domain found in the active site regions of bacterial DnaG-type primases and their homologs. Primases synthesize RNA primers for the initiation of DNA replication. DnaG type primases are often closely associated with DNA helicases in primosome assemblies. The TOPRIM 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. The DXD motif may co-ordinate Mg2+, a cofactor required for full catalytic function. The prototypical bacterial primase. Escherichia coli DnaG is a single subunit enzyme.