By its functional interaction with a RecA polymer, the mutagenic UmuD'C complex possesses an antirecombination activity. We show here that MucA'B, a functional homolog of the UmuD'C complex, inhibits homologous recombination as well. In F- recipients expressing MucA'B from a Ptac promoter, Hfr x F- recombination decreased with increasing MucA'B concentrations down to 50-fold. In damage-induced pKM101-containing cells expressing MucA'B from the native promoter, recombination between a UV-damaged F lac plasmid and homologous chromosomal DNA decreased 10-fold. Overexpression of MucA'B together with UmuD'C resulted in a synergistic inhibition of recombination. RecA[UmuR] proteins, which are resistant to UmuD'C inhibition of recombination, are inhibited by MucA'B while promoting MucA'B-promoted mutagenesis efficiently. The data suggest that MucA'B and UmuD'C contact a RecA polymer at distinct sites. The MucA'B complex was more active than UmuD'C in promoting UV mutagenesis, yet it did not inhibit recombination more than UmuD'C does. The enhanced mutagenic potential of MucA'B may result from its inherent superior capacity to assist DNA polymerase in trans-lesion synthesis. In the course of this work, we found that the natural plasmid pKM101 expresses around 45,000 MucA and 13,000 MucB molecules per lexA(Def) cell devoid of LexA. These molecular Muc concentrations are far above those of the chromosomally encoded Umu counterparts. Plasmid pKM101 belongs to a family of broad-host-range conjugative plasmids. The elevated levels of the Muc proteins might be required for successful installation of pKM101-like plasmids into a variety of host cells.