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The RpoS sigma factor protein of  Escherichia coli  RNA polymerase is the master transcriptional regulator of physiological responses to a variety of stresses. This stress response comes at the expense of scavenging for scarce resources, causing a trade-off between stress tolerance and nutrient acquisition. This trade-off favors non-functional  rpoS  alleles in nutrient-poor environments. We used experimental evolution to explore how natural selection modifies the regulatory network of strains lacking RpoS when they evolve in an osmotically stressful environment. We found that strains lacking RpoS adapt less variably, in terms of both fitness increase and changes in patterns of transcription, than strains with functional RpoS. This phenotypic uniformity was caused by the same adaptive mutation in every independent population: the insertion of IS 10  into the promoter of the  otsBA  operon. OtsA and OtsB are required to synthesize the osmoprotectant trehalose, and transcription of  otsBA  requires RpoS in the wild-type genetic background. The evolved IS 10  insertion rewires expression of  otsBA  from RpoS-dependent to RpoS-independent, allowing for partial restoration of wild-type response to osmotic stress. Our results show that the regulatory networks of bacteria can evolve new structures in ways that are both rapid and repeatable.

Compensatory Evolution of Gene Regulation in Response to Stress by Escherichia coli Lacking RpoS