Plasmid-dosage effects on ultraviolet, visible light and methyl methanesulfonate mutagenesis in Salmonella typhimurium.

Salmonella typhimurium LT2 strains bearing plasmids pKM101, R64 or pColIb-P9 demonstrated enhanced UV survival when compared with strains not bearing plasmids. A strain of S. typhimurium bearing both pKM101 and pColIb-P9 survived UV irradiation slightly better than either of the single-plasmid strains. Spontaneous reversion of the hisG46 and trpE8 missense alleles was enhanced in each single-plasmid strain, and for the dual-plasmid strain containing pKM101 and pColIb-P9 enhancement represented a near additivity of the response seen for the single-plasmid strains. Following exposure to UV or visible-light irradiation, reversion of hisG46 and trpE8 was also enhanced in each single-plasmid strain, but quantitatively greater in the dual-plasmid strain and was equal to or slightly greater than additive the responses of the single-plasmid strains. In contrast to visible-light irradiation, UV exposure resulted in two phenotypic Trp+-revertant classes. One Trp+ class, having normal colony size (2.0 mm) and similar in number to His+ revertants, was comprised of intragenic revertants of trpE8, while the predominant Trp+ class, having smaller colony size (0.8 mm), represented intergenic suppressor revertants, illuminating the differences in mutation and/or repair specificity for UV and visible-light exposure. Methyl methane-sulfonate (MMS)-induced reversion of hisG46 was similar in effect to that seen with UV or visible-light irradiation. Plasmids pKM101 or pColIb-P9 enhanced the frequency of hisG46 reversion, while a more than additive response was seen in a strain with both plasmids. Furthermore, MMS-induced reversion of hisG46 was also observed to be greatest in a strain bearing plasmid R64 (incompatibility group I alpha) and pKM101, when compared with single-plasmid strains bearing either R64 or pKM101.