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Journal of Bacteriology, July 2008, p. 5087-5094, Vol. 190, No. 14
0021-9193/08/$08.00+0     doi:10.1128/JB.01976-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Impact of Individual Mutations on Increased Fitness in Adaptively Evolved Strains of Escherichia coli{triangledown} ,{dagger}

M. Kenyon Applebee,1 Markus J. Herrgård,2 and Bernhard Ø. Palsson2*

Department of Chemistry and Biochemistry, University of California-San Diego, La Jolla, California 92093-0332,1 Department of Bioengineering, University of California-San Diego, La Jolla, California 92093-04122

Received 19 December 2007/ Accepted 7 May 2008

We measured the relative fitness among a set of experimentally evolved Escherichia coli strains differing by a small number of adaptive mutations by directly measuring allelic frequencies in head-to-head competitions using a mass spectrometry-based method. We compared the relative effects of mutations to the same or similar genes acquired in multiple strains when expressed in allele replacement strains. We found that the strongest determinant of fitness among the evolved strains was the impact of beneficial mutations to the RNA polymerase β and β' subunit genes. We also identified several examples of epistatic interactions between rpoB/C and glpK mutations and identified two other mutations that are beneficial only in the presence of previously acquired mutations but that have little or no adaptive benefit to the wild-type strain. Allele frequency estimation is shown to be a highly sensitive method for measuring selection rates during competitions between strains differing by as little as a single-nucleotide polymorphism and may be of great use for investigating epistatic interactions.

* Corresponding author. Mailing address: Department of Bioengineering, University of California, San Diego, 9500 Gilman Dr., MC 0412, La Jolla, CA 92093-0412. Phone: (858) 534-5668. Fax: (858) 822-3120. E-mail: palsson{at}ucsd.edu

{triangledown} Published ahead of print on 16 May 2008.

{dagger} Supplemental material for this article may be found at http://jb.asm.org/.

Journal of Bacteriology, July 2008, p. 5087-5094, Vol. 190, No. 14
0021-9193/08/$08.00+0     doi:10.1128/JB.01976-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.





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