Osmolytes Contribute to pH Homeostasis of Escherichia coli

Abstract

BackgroundCytoplasmic pH homeostasis in Escherichia coli includes numerous mechanisms involving pH-dependent catabolism and ion fluxes. An important contributor is transmembrane K+ flux, but the actual basis of K+ compensation for pH stress remains unclear. Osmoprotection could mediate the pH protection afforded by K+ and other osmolytes.Methods and Principal FindingsThe cytoplasmic pH of E. coli K-12 strains was measured by GFPmut3 fluorimetry. The wild-type strain Frag1 was exposed to rapid external acidification by HCl addition. Recovery of cytoplasmic pH was enhanced equally by supplementation with NaCl, KCl, proline, or sucrose. A triple mutant strain TK2420 defective for the Kdp, Trk and Kup K+ uptake systems requires exogenous K+ for steady-state pH homeostasis and for recovery from sudden acid shift. The K+ requirement however was partly compensated by supplementation with NaCl, choline chloride, proline, or sucrose. Thus, the K+ requirement was mediated in part by osmolarity, possibly by relieving osmotic stress which interacts with pH stress. The rapid addition of KCl to strain TK2420 suspended at external pH 5.6 caused a transient decrease in cytoplasmic pH, followed by slow recovery to an elevated steady-state pH. In the presence of 150 mM KCl, however, rapid addition of another 150 mM KCl caused a transient increase in cytoplasmic pH. These transient effects may arise from secondary K+ fluxes occurring through other transport processes in the TK2420 strain.ConclusionsDiverse osmolytes including NaCl, KCl, proline, or sucrose contribute to cytoplasmic pH homeostasis in E. coli, and increase the recovery from rapid acid shift. Osmolytes other than K+ restore partial pH homeostasis in a strain deleted for K+ transport.

Trk2 Potassium Transport System in Streptococcus mutans and Its Role in Potassium Homeostasis, Biofilm Formation, and Stress Tolerance

“…Each of the K ϩ transport systems Trk1, Trk2, GlnQHMP, and Kch was tested for its role in K ϩ uptake by complementing the E. coli strain TK2420 with these systems. E. coli TK2420 is a wellestablished model organism to characterize K ϩ transporters because of its lack of constitutive transporters and inability to transport K ϩ (31,(61)(62)(63). We observed that complementing E. coli TK2420 with Trk1, Trk2, or GlnQHMP facilitated significant K ϩ acquisition and enabled normal growth in low K ϩ concentrations and under stress conditions imposed by high salt concentrations and low pH.…”

Escherichia coli YqjA, a Member of the Conserved DedA/Tvp38 Membrane Protein Family, Is a Putative Osmosensing Transporter Required for Growth at Alkaline pH

“…5 and 6). Each of these agents is capable of imposing increased osmotic pressure on bacteria (28)(29)(30). One interpretation of this surprising result is that YqjA may additionally possess osmosensing capabilities.…”

Transcriptomic analysis reveals how a lack of potassium ions increases Sulfolobus acidocaldarius sensitivity to pH changes

“…In contrast, the absence of potassium resulted in a greater response with 75 statistically different RNA transcript ratios (Figs 3 and 4a and Supplemental Files 4 and 5). A sodium/proline symporter (Saci_0383) increased RNA transcripts in the absence of sodium, likely due to its role in cation homeostasis (Kitko et al, 2010). Increased RNA transcripts were observed for the subunit D of archaeal succinate dehydrogenase (Saci_0979) in the absence of sodium, while RNA transcripts for the cytochrome b homologue (soxC) of the Sox-ABCD complex (Saci_2087) increased under full salt.…”