title: Different characteristics distinguish early versus late arising adaptive mutations in Escherichia coli FC40.
abstract: The Escherichia coli strain FC40 has frequently been employed to investigate the mechanism of adaptive mutations. The strain cannot utilize lactose due to a +1 frameshift mutation that reduces beta-galactosidase to about 1% of normal levels. Cells undergo a high rate of mutation from Lac- to Lac+ when cells are grown with lactose as the sole energy source. Almost all Lac+ colonies arising 3-6 days after plating result from a base pair deletion in runs of iterated base pairs within a 130-bp target region. In this study we characterized Lac+ colonies arising 3-10 days after plating. Temperature gradient gel electrophoresis (TGGE) was used to detect mutations in the target region as a function of the day a colony appears. TGGE results confirmed the occurrence of mutations within the target region in 36 of 37 FC40 Lac+ colonies arising on days 3-7. However, mutations in this region were not detected in 23 of 37 Lac+ colonies arising from days 8-10. Sequencing data verified the TGGE results. Half of the Lac+ mutants arising on days 8-10 with no base pair change in the target region were unstable and exhibited a Lac- phenotype after successive growth cycles in rich medium. The results suggest that amplification of the lac operon region is a common factor in late arising colonies, and that different characteristics distinguish early and late arising Lac+ colonies.
authors.affiliation: Georgia Institute of Technology, School of Biology, Atlanta, GA 30332, USA.
authors.forename: S C
authors.initials: SC
authors.lastname: Powell
authors.forename: R M
authors.initials: RM
authors.lastname: Wartell
mesh_terms: D000220:Adaptation, Biological; D003470:Culture Media; D004252:DNA Mutational Analysis; D004269:DNA, Bacterial / Q000032:analysis; D004926:Escherichia coli / Q000235:genetics*; D007763:Lac Operon; D009154:Mutation; D016133:Polymerase Chain Reaction
chemical_list: D003470:Culture Media; D004269:DNA, Bacterial