The rapid evolution of flagellar ion-selectivity in experimental populations of E. coli

Abstract

Determining which cellular processes facilitate adaptation requires a tractable experimental model where an environmental cue can generate variants which rescue function. The Bacterial Flagellar Motor (BFM) is an excellent candidate - an ancient and highly conserved molecular complex for propulsion which navigates bacteria towards favourable environments. In most species, rotation is powered by H+ or Na+ ion transit through the torque-generating stator subunit of the motor complex. The ion that drives the rotor has changed over evolutionary timescales but the molecular basis of this selectivity remains unknown. Here we used CRISPR engineering to replace the native Escherichia coli H+-powered stator with Na+-powered stator genes and report the rapid and spontaneous reversion of our edit in a low sodium environment. We followed the evolution of the stators during their reversion to H+-powered motility and used whole genome and transcriptome sequencing to identify both flagellar- and non-flagellar-associated genes involved in the cells adaptation. Our transplant of an unfit protein and the cells rapid response to this edit demonstrates the adaptability of the stator subunit and highlights the hierarchical modularity of the flagellar motor.