Salmonella typhimurium LT2 mutation affecting the deletion of resistance determinants on R plasmids

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Abstract
Plasmid Rms312, specifying resistance to tetracycline (Tc), chloramphenicol (Cm), streptomycin (Sm), sulfonamide (Su) and mercury chloride (Mer), deletes both Tc and Cm Sm Su Mer determinants at a high frequency in S. typhimurium LT2. S. typhimurium mutants that were stable carriers of Rms312 were isolated by alternate culture of R-bearing cells in a medium containing either tetracycline or chloramphenicol. In one of these mutants the deletion frequency of drug resistance determinants was decreased by about 100-fold not only in Rms312, but also in R100, R1 and R6-5. This mutation caused a slight reduction of UV resistance but did not affect generalized genetic recombination, indicating that the mutation is different from recA. The mutation, designated dor (deletion of r-determinants), was mapped to a position near 57 units in the new linkage map of S. typhimurium LT2. The dor mutation had no effect on IS [insertion sequence] 1-mediated illegitimate deletion, indicating that the dor mutation is different from the del mutation described by Nevers and Saedler.