Plasmid-encoded MucB protein is a DNA polymerase (pol RI) specialized for lesion bypass in the presence of MucA ', RecA, and SSB

Replication through damaged sites in DNA requires in Escherichia coli the S OS stress-inducible DNA polymerase V (UmuC). which is specialized for lesio n bypass. Homologs of the umuC gene were found on native conjugative plasmi ds, which often carry multiple antibiotic-resistant genes. MucB is a UmuC h omolog present on plasmid R46, and its variant plasmid pKM101 has been intr oduced into Salmonella strains for use in the Ames test for mutagens. Using a translesion replication assay based on a gapped plasmid carrying a site- specific synthetic abasic site in the single-stranded DNA region, we show t hat MucB is a DNA polymerase, termed pol RI. which is specialized for lesio n bypass. The activity of pol RI requires the plasmid-encoded MucA' protein and the E. coli RecA and single-strand DNA binding proteins. Elimination o f any of the proteins from the reaction abolished lesion bypass and polymer ase activity. The unprocessed MucA could not substitute for MucA' in the by pass reaction. The presence of a lesion bypass DNA polymerase on a native c onjugative plasmid. which has a broad host range specificity and carries mu ltiple antibiotic-resistant genes, raises the possibility that mutagenesis caused by pol RI plays a role in the spreading of antibiotic resistance amo ng bacterial pathogens.