Figure 5. Effect of ΔrnhA and ΔrnhB on spontaneous mutagenesis in recA730 lexA(Def) ΔumuDC ΔdinB strains expressing pol V variants.

Spontaneous mutagenesis was measured by assaying reversion of the hisG4 ochre allele (leading to histidine prototophy) as described in Materials and Methods . The average number of His⁺ revertants per plate ± standard error of the mean is indicated in the table. Since the extent of mutagenesis promoted by wild-type pol V differed in the various strains, we have expressed the level of mutagenesis promoted by the variants as a percentage of wild-type mutagenesis. As clearly observed, umuC_Y11A-dependent mutagenesis increased in the ΔrnhB strain and was further elevated in the ΔrnhA ΔrnhB double mutant. In contrast, umuC_F10L gave consistently low levels of mutagenesis in all strains, and umuC_Y11F higher than wild-type levels in all strains.