A Bacillus Flagellar Motor That Can Use Both Na+ and K+ as a Coupling Ion Is Converted by a Single Mutation to Use Only Na+

Naoya Terahara,Masahiro Ito,Motohiko Sano,Arnold Driessen

doi:10.1371/journal.pone.0046248

Abstract

In bacteria, the sodium ion (Na+) cycle plays a critical role in negotiating the challenges of an extremely alkaline and sodium-rich environment. Alkaliphilic bacteria that grow optimally at high pH values use Na+ for solute uptake and flagellar rotation because the proton (H+) motive force is insufficient for use at extremely alkaline pH. Only three types of electrically driven rotary motors exist in nature: the F-type ATPase, the V-type ATPase, and the bacterial flagellar motor. Until now, only H+ and Na+ have been reported as coupling ions for these motors. Here, we report that the alkaliphilic bacterium Bacillus alcalophilus Vedder 1934 can grow not only under a Na+-rich and potassium ion (K+)-poor condition but also under the opposite condition in an extremely alkaline environment. In this organism, swimming performance depends on concentrations of Na+, K+ or Rb+. In the absence of Na+, swimming behavior is clearly K+- dependent. This pattern was confirmed in swimming assays of stator-less Bacillus subtilis and Escherichia coli mutants expressing MotPS from B. alcalophilus (BA-MotPS). Furthermore, a single mutation in BA-MotS was identified that converted the naturally bi-functional BA-MotPS to stators that cannot use K+ or Rb+. This is the first report that describes a flagellar motor that can use K+ and Rb+ as coupling ions. The finding will affect the understanding of the operating principles of flagellar motors and the molecular mechanisms of ion selectivity, the field of the evolution of environmental changes and stresses, and areas of nanotechnology.

Highlights

Bacterial flagella act as rigid propellers for cell locomotion in liquid and on surfaces, and their rotation is powered by electrochemical gradients of either protons (H+) or sodium ions (Na+) across the bacterial cytoplasmic membrane [1,2]
In 2008, we reported that alkaliphilic Bacillus clausii KSM-K16 utilises a novel form of dual-ion coupling for motility, in which a single MotAB stator uses both sodium and protons at different pH ranges [14]
As measured using flame photometry, MYE medium is rich in sodium ions (189 mM) and poor in potassium ions (5.5 mM), and KMYE medium is rich in potassium ions (210 mM) and poor in sodium ions (0.7 mM)

Summary

Introduction

Bacterial flagella act as rigid propellers for cell locomotion in liquid and on surfaces, and their rotation is powered by electrochemical gradients of either protons (H+) or sodium ions (Na+) across the bacterial cytoplasmic membrane [1,2]. In 2008, we reported that alkaliphilic Bacillus clausii KSM-K16 utilises a novel form of dual-ion coupling for motility, in which a single MotAB stator uses both sodium and protons at different pH ranges [14]. No stators that can use potassium ions, which is reported here for the MotPS system of alkaliphilic Bacillus alcalophilus Vedder 1934, have previously been described. This strain, which was isolated from human feces in 1934, is a classic alkaliphilic strain because it was among the very first alkaliphilic ‘‘extremophiles’’ to be described in the literature [16]

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