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Originally published as Genetics Published Articles Ahead of Print on August 30, 2008.
Genetics, Vol. 180, 431-443, September 2008, Copyright © 2008
doi:10.1534/genetics.108.091330
Genetic Basis of Evolutionary Adaptation by Escherichia coli to Stressful Cycles of Freezing, Thawing and Growth
Sean C. Sleight*,1,
Christian Orlic*,
Dominique Schneider
,
and
Richard E. Lenski*
* Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, Michigan 48824, Laboratoire Adaptation et Pathogénie des Micro-organismes, Université Joseph Fourier Grenoble 1, F-38042 Grenoble Cedex 9, France and CNRS UMR 5163, F-38042 Grenoble Cedex 9, France
1 Corresponding author: Department of Bioengineering, University of Washington, Seattle, WA 98195.
E-mail: sleight{at}u.washington.edu
Microbial evolution experiments offer a powerful approach for coupling changes in complex phenotypes, including fitness and its components, with specific mutations. Here we investigate mutations substituted in 15 lines of Escherichia coli that evolved for 1000 generations under freeze–thaw–growth (FTG) conditions. To investigate the genetic basis of their improvements, we screened many of the lines for mutations involving insertion sequence (IS) elements and identified two genes where multiple lines had similar mutations. Three lines had IS150 insertions in cls, which encodes cardiolipin synthase, and 8 lines had IS150 insertions in the uspA-uspB intergenic region, encoding two universal stress proteins. Another line had an 11-bp deletion mutation in the cls gene. Strain reconstructions and competitions demonstrated that this deletion is beneficial under the FTG regime in its evolved genetic background. Further experiments showed that this cls mutation helps maintain membrane fluidity after freezing and thawing and improves freeze–thaw (FT) survival. Reconstruction of isogenic strains also showed that the IS150 insertions in uspA/B are beneficial under the FTG regime. The evolved insertions reduce uspB transcription and increase both FT survival and recovery, but the physiological mechanism for this fitness improvement remains unknown.
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Genetics 2008 180: NP.
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