Antimutagenic Effect of umuD Mutant Plasmids: Isolation and Characterization of umuD Mutants Reduced in their Ability to Promote UV Mutagenesis in Escherichia Coli

umuD and umuC are essential genes for UV and most chemical mutagenesis in Escherichia coli (4, 15, 18). Both umuD and umuC mutants are virtually nonmutable with ultraviolet (UV) and a variety of chemicals (6, 17). The umuD and umuC genes are organized as an operon (4, 15) and encode proteins of 15.0 and 47.7 kilodaltons (kDa), respectively (7, 12). The umuDC operon is repressed by the LexA protein (1, 4) and regulated as a part of SOS response of E. coli (10, 18, 19). SOS response occurs by activated RecA mediating the proteolytic cleavage of a bond between Ala84 and Gly85 of LexA (9), apparently facilitating a conditional autodigestion of LexA (16).