Evolution of dnaQ, the gene encoding the editing 3′ to 5′ exonuclease subunit of DNA polymerase III holoenzyme in Gram‐negative bacteria

The nucleotide sequences of the dnaQ genes from Salmonella typhimurium and Buchnera aphidicola, encoding the epsilon-subunit of the DNA polymerase III holoenzyme, have been determined. The Salmonella typhimurium dnaQ protein consists of 243 amino acid residues with a calculated molecular weight of 27224. The Buchnera aphidicola dnaQ protein contains 233 amino acid residues with a calculated molecular weight of 27170. A multiple sequence alignment of the amino acid sequences of the dnaQ proteins and those of DNA polymerase IIIs from Gram-positive bacteria produced six homologous segments. These homologous segments contain highly conserved amino acid sequence motifs involved in catalytically important metal ion bindings (ligands 1, 2 and 3). However, metal ligand 4 is found to be altered in the 3'-5' exonuclease domain of the family C DNA polymerases and dnaQ proteins in Gram-negative bacteria. From these results, we propose that the last common ancestor of the dnaQ gene of Gram-negative bacteria and the DNA polymerase III gene (pol C gene) of Gram-positive bacteria was a single gene containing both 3'-5' exonuclease and DNA polymerase domains and then the dnaQ gene separated from the polymerase gene in Gram-negative bacteria.