title: Plasmid copy number underlies adaptive mutability in bacteria.
abstract: The origin of mutations under selection has been intensively studied using the Cairns-Foster system, in which cells of an Escherichia coli lac mutant are plated on lactose and give rise to 100 Lac+ revertants over several days. These revertants have been attributed variously to stress-induced mutagenesis of nongrowing cells or to selective improvement of preexisting weakly Lac+ cells with no mutagenesis. Most revertant colonies (90%) contain stably Lac+ cells, while others (10%) contain cells with an unstable amplification of the leaky mutant lac allele. Evidence is presented that both stable and unstable Lac+ revertant colonies are initiated by preexisting cells with multiple copies of the F'lac plasmid, which carries the mutant lac allele. The tetracycline analog anhydrotetracycline (AnTc) inhibits growth of cells with multiple copies of the tetA gene. Populations with tetA on their F'lac plasmid include rare cells with an elevated plasmid copy number and multiple copies of both the tetA and lac genes. Pregrowth of such populations with AnTc reduces the number of cells with multiple F'lac copies and consequently the number of Lac+ colonies appearing under selection. Revertant yield is restored rapidly by a few generations of growth without AnTc. We suggest that preexisting cells with multiple F'lac copies divide very little under selection but have enough energy to replicate their F'lac plasmids repeatedly until reversion initiates a stable Lac+ colony. Preexisting cells whose high-copy plasmid includes an internal lac duplication grow under selection and produce an unstable Lac+ colony. In this model, all revertant colonies are initiated by preexisting cells and cannot be stress induced.
authors.affiliation: Department of Microbiology and Molecular Genetics, University of California, Davis, California 95616.
authors.forename: Emiko
authors.initials: E
authors.lastname: Sano
authors.affiliation: Department of Microbiology and Molecular Genetics, University of California, Davis, California 95616.
authors.forename: Sophie
authors.initials: S
authors.lastname: Maisnier-Patin
authors.affiliation: Department of Microbiology and Molecular Genetics, University of California, Davis, California 95616.
authors.forename: John Paul
authors.initials: JP
authors.lastname: Aboubechara
authors.affiliation: Department of Microbiology and Molecular Genetics, University of California, Davis, California 95616.
authors.forename: Semarhy
authors.initials: S
authors.lastname: Quiñones-Soto
authors.affiliation: Department of Microbiology and Molecular Genetics, University of California, Davis, California 95616 jrroth@ucdavis.edu.
authors.forename: John R
authors.initials: JR
authors.lastname: Roth
mesh_terms: D000220:Adaptation, Biological / Q000187:drug effects / Q000235:genetics*; D056915:DNA Copy Number Variations / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics* / Q000254:growth & development; D005784:Gene Amplification; D018628:Gene Dosage; D020440:Gene Duplication / Q000187:drug effects; D005798:Genes, Bacterial; D007763:Lac Operon; D016254:Mutagenesis, Insertional; D009154:Mutation / Q000235:genetics*; D010957:Plasmids / Q000235:genetics*; D012486:Salmonella typhimurium / Q000235:genetics* / Q000254:growth & development; D013754:Tetracyclines / Q000494:pharmacology
chemical_list: D013754:Tetracyclines; C016229:4-epianhydrotetracycline
keywords: adaptive mutation; gene amplification; mutation under selection; selection; stress-induced mutagenesis