Genetics, Vol. 163, 1483-1496, April 2003, Copyright © 2003

Regulating General Mutation Rates: Examination of the Hypermutable State Model for Cairnsian Adaptive Mutation

John R. Rotha, Eric Kofoida, Frederick P. Rothb, Otto G. Bergc, Jon Segera, and Dan I. Anderssond
a Department of Biology, University of Utah, Salt Lake City, Utah 84122,
b Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115,
c Department of Molecular Evolution, Evolution Biology Centre, Uppsala University, SE-75236 Uppsala, Sweden
d Department of Bacteriology, Swedish Institute for Infectious Disease Control, S-171 82 Solna, Sweden

Corresponding author: John R. Roth, University of California, Davis, CA 95616., jrroth{at}ucdavis.edu (E-mail)

Communicating editor: M. W. FELDMAN

In the lac adaptive mutation system of Cairns, selected mutant colonies but not unselected mutant types appear to arise from a nongrowing population of Escherichia coli. The general mutagenesis suffered by the selected mutants has been interpreted as support for the idea that E. coli possesses an evolved (and therefore beneficial) mechanism that increases the mutation rate in response to stress (the hypermutable state model, HSM). This mechanism is proposed to allow faster genetic adaptation to stressful conditions and to explain why mutations appear directed to useful sites. Analysis of the HSM reveals that it requires implausibly intense mutagenesis (105 times the unselected rate) and even then cannot account for the behavior of the Cairns system. The assumptions of the HSM predict that selected revertants will carry an average of eight deleterious null mutations and thus seem unlikely to be successful in long-term evolution. The experimentally observed 35-fold increase in the level of general mutagenesis cannot account for even one Lac+ revertant from a mutagenized subpopulation of 105 cells (the number proposed to enter the hypermutable state). We conclude that temporary general mutagenesis during stress is unlikely to provide a long-term selective advantage in this or any similar genetic system.





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