Structure and function of dnaQ and mutD mutators of Escherichia coli

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 (ε 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) and¹⁶⁴Ala (GCA)→Val (GTA), which were designated the mutD52 and mutD51 mutations, respectively. Construction of chimaeric genes carrying one or two of these mutations revealed: (1) either mutD51 or mutD52 alone causes the dominant mutator phenotype when present in a multi-copy plasmid; (2)mutD51, but not mutD52, exerts the dominant mutator phenotype when present in a low-copy plasmid; (3) 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.