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Iterative profile searches and structural modeling show that bacterial DnaG-type primases, small primase-like proteins from bacteria and archaea, type IA and type II topoisomerases, bacterial and archaeal nucleases of the OLD family and bacterial DNA repair proteins of the RecR/M family contain a common domain, designated Toprim (topoisomerase-primase) domain. The domain consists of approximately 100 amino acids and has two conserved motifs, one of which centers at a conserved glutamate and the other one at two conserved aspartates (DxD). Examination of the structure of Topo IA and Topo II and modeling of the Toprim domains of the primases reveal a compact beta/alpha fold, with the conserved negatively charged residues juxtaposed, and inserts seen in Topo IA and Topo II. The conserved glutamate may act as a general base in nucleotide polymerization by primases and in strand rejoining by topoisomerases and as a general acid in strand cleavage by topoisomerases and nucleases. The role of this glutamate in catalysis is supported by site-directed mutagenesis data on primases and Topo IA. The DxD motif may coordinate Mg2+that is required for the activity of all Toprim-containing enzymes. The common ancestor of all life forms could encode a prototype Toprim enzyme that might have had both nucleotidyl transferase and polynucleotide cleaving activity.