The Escherichia coli Umu proteins are best characterized by their role in damage inducible mutagenesis. Recently, we discovered that the intracellular levels of the UmuD and UmuC proteins are kept to a minimum by the Lon serine protease. Studies with the Salmonella typhimurium UmuD protein (which is 73% homologous with its E. coli counterpart) revealed that it too is degraded by Lon, suggesting that both UmuD proteins share conserved structural motifs. In contrast, E. coli UmuD' is removed from the cell by the ClpXP serine protease, but only when it is in a heterodimer complex with UmuD. We have generated deletion mutants of UmuD' and have coexpressed the mutant proteins with UmuD1 (a non-cleavable UmuD protein). By assaying the sensitivity of the mutant UmuD'-UmuD1 complex to ClpXP, we have been able to map regions of UmuD' that appear essential for efficient UmuD'-UmuD heterodimer formation. Previous experiments have suggested that the in vivo posttranslational processing of UmuD to UmuD' is inefficient. We have, however, discovered that limited cleavage occurs in an undamaged cell, but that these small amounts of UmuD' are rapidly degraded by ClpXP, thus giving rise to the appearance of inefficient cleavage. The ClpXP protease therefore plays dual roles in regulating SOS mutagenesis: it keeps the basal levels of UmuD' to a minimum in undamaged cells but it also acts in damaged cells to reduce the elevated levels of mutagenically active UmuD' protein, thereby returning the cell to a resting non-mutable state.