Fig 4.

Spectra of spontaneously arising rpoB mutations in recA730 lexA(Def) ΔdinB ΔumuDC ΔmutLrnhB +/− strains expressing umuC_Y11A. A. polA _D424A strains deficient in pol I 3′→5′ exonuclease activity. The arrows indicate mutagenic hot spots within the rpoB. B. polA _ΔC strains deficient in pol Ipolymerase activity. Approximately 300 Rif ^R mutants were analyzed for each strain (Table 2). The forward mutation rates were assayed by measuring resistance to rifampicin and were in the range of 3–5 × 10⁻⁶. The spectra in the rnhB+ and ΔrnhB strains differ in both the types of mutations and locations of mutagenic hot-spots. Because of defects in MMR, all the spectra are dominated by transition mutations. However, since low-fidelity pol V makes a significant number of transversion mutations compared to other E. coli DNA polymerases [34, 37, 52], the number of umuC_Y11A-dependent transversions in the rnhB strains is higher than in the rnhB⁺ strains (see Table 2). The shift in the types of mutations observed is significantly more pronounced in the polA_ΔC strains. The rnhB⁺ strains undergo active RER; as a consequence, they have a reduced number of pol V-specific transversions, and the spectrum generated in these strains reflect uncorrected errors made by pol I (panel A) or pol III (panel B) during the DNA re-synthesis step of RER.