Phenotypic instability of Salmonella typhimurium tester strains: an example of a plasmid-enhanced genetic drift?

Strains derived from Salmonella typhimurium LT-2, which are used as tester strains in mutagenicity assays, show significant changes in biochemical phenotypes. The presence of plasmid pKM101 in these strains greatly increases both the frequency of these shifts as well as the spectrum of phenotypes involved. It is suggested that in plasmid-free strains these variations reflect the effects of endogeneously induced mutations which are amplified in the absence of a functional uvrB gene product. In plasmid-containing strains this genetic drift may be promoted further by the pKM101-coded error-prone DNA repair system. The observation of a plasmid-mediated genetic drift lends support to the suggestion that transposons may contribute to the carcinogenic process.