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Plasmid copy number underlies adaptive mutability in bacteria.

Emiko Sano1, Sophie Maisnier-Patin1, John Paul Aboubechara1

  • 1Department of Microbiology and Molecular Genetics, University of California, Davis, California 95616.

Genetics
|September 1, 2014
PubMed
Summary

The Cairns-Foster system study reveals that Lac+ revertants in Escherichia coli arise from preexisting cells with multiple F'lac plasmids, not stress-induced mutagenesis. This challenges previous theories on mutation origins under selection.

Keywords:
adaptive mutationgene amplificationmutation under selectionselectionstress-induced mutagenesis

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Area of Science:

  • Microbiology
  • Genetics
  • Molecular Biology

Background:

  • The Cairns-Foster system in Escherichia coli is a model for studying mutation origins under selection.
  • Revertants (Lac+) have been attributed to stress-induced mutagenesis or selection of pre-existing variants.

Purpose of the Study:

  • To investigate the cellular mechanisms initiating Lac+ revertant colonies in Escherichia coli under selection.
  • To differentiate between stress-induced mutagenesis and selection of pre-existing cells.

Main Methods:

  • Utilized the Cairns-Foster system with Escherichia coli lac mutants on lactose plates.
  • Employed anhydrotetracycline (AnTc) to inhibit growth of cells with multiple F'lac plasmids carrying tetA.
  • Analyzed stable and unstable Lac+ revertant colony formation.

Main Results:

  • Both stable (90%) and unstable (10%) Lac+ revertant colonies are initiated by pre-existing cells with multiple F'lac copies.
  • Anhydrotetracycline (AnTc) treatment reduced revertant yield by targeting cells with elevated F'lac copy numbers.
  • Revertant yield was restored upon removal of AnTc, indicating a reversible mechanism.

Conclusions:

  • All Lac+ revertant colonies originate from pre-existing cells with multiple F'lac plasmids.
  • The findings refute the model of stress-induced mutagenesis in nongrowing cells for this system.
  • Cellular mechanisms involve plasmid replication and potential lac gene duplication within high-copy plasmids.