Compensating the fitness costs of synonymous mutations
Anna Knöppel, Joakim Näsvall and Dan I. Andersson*
- Author Affiliations
Department of Medical Biochemistry and Microbiology, Uppsala University, Sweden
↵* for correspondence:
email: Dan.Andersson@imbim.uu.se,
phone: +46 018 4714175
Received October 29, 2015. Revision received December 16, 2015. Revision received January 21, 2016. Accepted January 22, 2016.
Abstract
Synonymous mutations do not change the sequence of the polypeptide but they may still influence fitness. We investigated in Salmonella enterica how four synonymous mutations in the rpsT gene (encoding ribosomal protein S20) reduce fitness (i.e. growth rate) and the mechanisms by which this cost can be genetically compensated. The reduced growth rates of the synonymous mutants were correlated with reduced levels of the rpsT transcript and S20 protein. In an adaptive evolution experiment these fitness impairments could be compensated by mutations that either caused up-regulation of S20 through increased gene dosage (due to duplications), increased transcription of the rpsT gene (due to an rpoD mutation or mutations in rpsT), or increased translation from the rpsT transcript (due to rpsT mutations). We suggest that the reduced levels of S20 in the synonymous mutants result in production of a defective subpopulation of 30S subunits lacking S20 that reduce protein synthesis and bacterial growth and that the compensatory mutations restore S20 levels and the number of functional ribosomes. Our results demonstrate how specific synonymous mutations can cause substantial fitness reductions and that many different types of intra- and extragenic compensatory mutations can efficiently restore fitness. Furthermore, our study highlights that also synonymous sites can be under strong selection, which may have implications for the use of dN/dS ratios as signature for selection.
© The Author(s) 2016. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution.
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