(A) Thiourea (TU) reduces Lac reversion dose-dependently. Representative experiment showing the effects of varying doses of TU on the yield of Lac+ mutant cfu of SMR4562. (B) Quantification of mutation rates from 3 experiments, days 3 to 7. Adding 25, 50, and 100 mM TU to plates reduced mutagenesis 62% ± 5.6%, 82% ± 5.6% and 90% ± 4.3% respectively (p = 8.3x10-5, 1.7x10-5, and 5.2x10-5, compared with 0mM TU, Student’s 2-tailed t-test). (C) TU eliminated the hypermutation seen in Δdps strain cells dose-dependently. (D) 2'2-bipyridine (bip) reduces MBR dose-dependently. Representative experiment. (E) Either 0.1 or 0.2 μM bip reduced mutagenesis by 23% ± 4.0% and by 40% ±7.8%, respectively (p = 0.007 and 0.02, Student’s 2-tailed t-test). * indicates significantly different from the samples with zero dose of TU or bip. Strains used: wild type, SMR4562; Δdps, PJH2608.