THE SSB-113 ALLELE SUPPRESSES THE DNAQ49 MUTATOR AND ALTERS DNA SUPERCOILING IN ESCHERICHIA-COLI

Mutations in the dnaQ gene, which encodes the proof-reading epsilon-su bunit of the DNA polymerase III holoenzyme, lead to a mutator phenotyp e caused by enhanced error rates during DNA replication. In this paper , we studied the influence of ssb mutations on the dnaQ49 mutator, bec ause of the involvement of SSB protein in DNA replication. We found th at the ssb-113 mutation suppresses the mutator phenotype of dnaQ49. Th e suppression effect resulted from an enhanced expression of the dnaQ4 9 allele as determined by experiments with gene fusions. S1 nuclease a nalysis revealed that the increased dnaQ expression is based on transc riptional activation of the dnaQP2 promoter. This seems to be the cons equence of an increased DNA supercoiling in the ssb-113 mutant, which also influenced further functions that are sensitive to alterations in DNA supercoiling. These results support the hypothesis that the expre ssion of the epsilon-subunit of DNA polymerase III may additionally be modulated by DNA supercoiling, and suggest a possible role for DNA to pology in mutagenesis.