Mutagenesis-enhancement by plasmids in mutagenesis tester strains

The plasmid pKM101 has played a very important role in the success of the Ames Salmonella test for carcinogens and mutagens. It was derived from the clinically isolated plasmid R46 by an in vivo deletion and confers upon its host both increased resistance to killing by UV irradiation and increased susceptibility to UV and chemical mutagenesis. pKM101 exerts its effects by coding for two genes mucA and mucB, which are analogs of the chromosomally-encoded genes umuD and umuC. The products of the umuD/C locus are required for UV and chemical mutagenesis. The mucA/B and umuD/C loci code for products of very similar molecular weight and the transcription organization of the two loci is identical. Expression of both the mucA/B and umuD/C loci is induced by DNA damage, and is regulated by the recA lexA control circuit.