Whatever the explanation, our results remain consistent with a role for MdtM in alkaline pH homeostasis in E. coli. In our growth experiments, the requirement for sodium or potassium ions for MdtM-mediated alkalitolerance suggests a mechanistic role for Na+ and K+ ions in MdtM activity and this

was confirmed by fluorescence-based activity assays performed at alkaline pH values (Figure 6). These assays showed that MdtM catalysed a Na+(K+)/H+ antiport that, in mTOR inhibitor drugs vivo, probably enables the exchange of internal monovalent metal cations for extracellular protons to maintain a stable internal pH, acid relative to outside, during exposure to alkaline environments. This conclusion was supported by our experiments that used BCECF fluorometry to measure cytoplasmic pH under different external alkaline pH conditions (Figure 10). The ability of MdtM to exchange either

Na+ or K+ cations for protons endows E. coli with the flexibility to respond effectively to changes in chemical composition of the environment at alkaline pH. When sodium is available, AZD5153 the Na+/H+ antiport activity of MdtM can permit growth. Under sodium-poor conditions, or when other Na+/H+ antiporters are disrupted, regulation of cytoplasmic pH by K+/H+ antiport activity of MdtM can contribute to alkaline pH homeostasis. Although the contribution of K+ concentration to pH homeostasis in E. coli is still unclear [6, 36], the K+/H+ antiport activity of MdtM may offer a mechanism for regulating cytoplasmic pH by utilising the outwardly-directed K+ gradient to drive proton capture during growth at (-)-p-Bromotetramisole Oxalate alkaline pH [5, 37]. Provided the rate of MdtM is slower than that of the systems that generate the PMF, and of the uptake systems that bring K+ into the cell, MdtM will not act as an uncoupler to dissipate the PMF. Furthermore, in alkaline environments,

the same K+/H+ antiport activity of MdtM has the potential to protect E. coli from the toxic effects of high intracellular concentrations of K+ and, therefore, to function also in K+ homeostasis. Just such a function was identified previously for the E. coli ChaA antiporter [12]. Additionally, and in contrast to MdfA, MdtM is capable of transporting lithium ions at alkaline pH (Figure 8B) and it may function physiologically in alkaline pH homeostasis when Li+ is present. This highlights further the subtle differences in function that exist between the closely-related MdfA and MdtM transporters, and that lessons learned from one cannot simply be imposed upon the other. As control of internal pH is, by definition, control of cytoplasmic proton concentration, the requirements of bacterial pH homeostasis dictate the relative magnitudes of the transmembrane proton gradient (ΔpH) and transmembrane electrical potential (Δψ), the two individual components that constitute the PMF.