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Salmonella typhimurium, especially its derivatives containing pKMlOl plasmid, has been widely used in the Ames test for the detection of environmental mutagens and carcinogens (1,12). It is known, however, that if the pKM101 plasmid is eliminated, S. typhimurium itself shows a much weaker mutagenic response to UV and some chemical mutagens than does Escherichia coli (12,22,25,32). In fact, certain potent base-change type mutagens, such as furylfuramide and aflatoxin B 1 , are nonmutagenic to 5. typhimurium in the absence of pKM101, whereas they are strongly mutagenic to S. typhimurium in the presence of pKM101 plasmid as well as to E. coli (12). The low mutability can be restored to levels comparable to E. coli by introducing the plasmid carrying the E. coli umuDC operon or the pKM101 plasmid carrying mucAB operon (11-13,32). Salmonella typhimurium has an SOS regulatory system which resembles that of E. coli (10,12,17,18,20,22). Thus, it was suggested that S. typhimurium is deficient in the function of umuDC operon, which plays an essential role in UV and most chemical mutagenesis in E. coli (7,28,33). |
