Temperature-dependent mutational specificity of an Escherichia coli mutator, dnaQ49, defective in 3'----5' exonuclease (proofreading) activity

Escherichia coli strains carrying the temperature-dependent dnaQ49 allele are strong mutators at 37 degrees C. Since the dnaQ49 gene encodes the epsilon subunit of DNA polymerase III, it is thought that the large number of errors results in part from impaired proofreading activity during DNA replication. We have examined dnaQ49-induced reversion patterns of defined trpA alleles to determine the kinds of errors produced by dnaQ49 at 30 degrees C and 37 degrees C. We found that at 37 degrees C dnaQ49 produced all types of base-pair substitutions in addition to frameshifts with transitions generally occurring more frequently than transversions. This generalized mutator activity is very similar to that displayed in rich medium by mutD5, another mutator allele at the dnaQ locus. However, when dnaQ49 strains were cultured at 30 degrees C, not only were reversion frequencies much lower than at 37 degrees C, but in addition, the spectrum was altered. Transversions became proportionally more prevalent in the reversion spectra at the lower temperature. We suggest the possibility that at 37 degrees C dnaQ49 results in defective proofreading and methyl-directed postreplicative mismatch repair, while at 30 degrees C mismatch repair is fully and proofreading partially restored.