The nucleotide sequences of the recessive dnaQ49 and the dominant mutD5 mutator were determined. The dnaQ49 mutator has a single base substitution in the dnaQ gene, thus causing one amino acid change, 96Val (GTG)----Gly (GGG), in the DnaQ protein (epsilon subunit of DNA polymerase III holoenzyme). The mutD5 mutator possesses two base substitutions in the same gene, resulting in two amino acid changes, 73Leu (TTG)----Trp (TGG) and 164Ala (GCA)----Val (GTA), which were designated the mutD52 and mutD51 mutations, respectively. Construction of chimaeric genes carrying one or two of these mutations revealed: either mutD51 or mutD52 alone causes the dominant mutator phenotype when present in a multi-copy plasmid; mutator phenotype when present in a low-copy plasmid; the dominant mutD51 mutator activity is suppressed by the dnaQ49 mutation when both mutations are present in the same gene. Based on these findings, we devised a model for the action of these mutators.