Modulation of mutagenesis involving precise excision of transposon Tn10

Precise excision of transposon Tn10 results in reversion of the Trp- phenotype to Trp+ in a trp-1014::Tn10 strain of Salmonella typhimurium, and also occurs at a markedly higher frequency in a strain carrying the temperature-sensitive polA7 allele. The frequency with which precise excision events occurs can be modified by the plating medium, results indicating that the great majority of mutants which arise on broth-supplemented or tryptophan-supplemented minimal media actually arise on the selective plating medium. Trp+ revertants (1000) arising from excision of Tn10 were purified by re-streaking for single colonies; none were found to retain the Tn10 encoded resistance to tetracycline. Yields of Trp+ revertants of the polA7 strain were consistently higher when glycerol rather than glucose was used as sole carbon source in the selective medium. Clean excision of Tn10 can also be increased by ultraviolet irradiation in (R) plasmid-free strains, and is further increased in strains carrying an N-group plasmid (R205, R46 or pKM101). Ultraviolet-induced precise excision of Tn10 also occurs at a much enhanced frequency in a strain with a deletion through the uvrB gene; in this case, however, the addition of plasmid pKM101 leads to a decrease in yields of ultraviolet-induced precise excision events.