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.