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.