Kup-mediated Cs⁺ uptake and Kdp-driven K⁺ uptake coordinate to promote cell growth during excess Cs⁺ conditions in Escherichia coli

The physiological effects of caesium (Cs) on living cells are poorly understood. Here, we examined the physiological role of Cs⁺ on the activity of the potassium transporters in E. coli. In the absence of potassium (K⁺), Kup-mediated Cs⁺ uptake partially supported cell growth, however, at a much lower rate than with sufficient K⁺. In K⁺-limited medium (0.1 mM), the presence of Cs⁺ (up to 25 mM) in the medium enhanced growth as much as control medium containing 1 mM K⁺. This effect depended on the maintenance of basal levels of intracellular K⁺ by other K⁺ uptake transporters. Higher amounts of K⁺ (1 mM) in the medium eliminated the positive effect of Cs⁺ on growth, and revealed the inhibitory effect of high Cs⁺ on the growth of wild-type E. coli. Cells lacking Kdp, TrkG and TrkH but expressing Kup grew less well when Cs⁺ was increased in the medium. A kdp mutant contained an increased ratio of Cs⁺/K⁺ in the presence of high Cs⁺ in the medium and consequently was strongly inhibited in growth. Taken together, under excess Cs⁺ conditions Kup-mediated Cs⁺ influx sustains cell growth, which is supported by intracellular K⁺ supplied by Kdp.