Efficiency of MucAB and Escherichia coli UmuDC proteins in quinolone and UV mutagenesis in Salmonella typhimurium: effect of MucA and UmuD processing

The role of MucAB and Escherichia coli UmuDC proteins in mutagenesis by 4-quinolone (4-Q) compared to that in UV mutagenesis has been studied in hisG428 Salmonella typhimurium strains. A low-copy plasmid carrying mucAB genes, but not umuDC, promotes reversion of the hisG428 mutation by the 4-Q ciprofloxacin. In contrast, a umuDC plasmid mediates the reversion of hisG428 by UV, although less efficiently than a mucAB one. In addition, a unique copy of mucAB genes is enough to promote UV mutagenesis, whereas, several copies of them are required to detect ciprofloxacin mutagenesis. Therefore, the mutagenic repair of quinolone damage by MucAB proteins is not a very efficient process. The presence of an umuD'C plasmid but not a mucA'B one, slightly increases the reversion of the hisG428 mutation by ciprofloxacin and this finding is further discussed. In contrast, MucA'B are still more active than UmuD'C proteins in UV mutagenesis. These results suggest that the enhanced processing of MucA compared to UmuD would not explain all functional differences between MucAB and UmuDC proteins in the error-prone DNA repair.