Restoration of the growth of Escherichia coli under K(+)-deficient conditions by Cs(+) incorporation via the K(+) transporter Kup

Biological incorporation of cesium ions (Cs(+)) has recently attracted significant attention in terms of the possible applications for bioremediation of radiocesium and their significant roles in biogeochemical cycling. Although high concentrations of Cs(+) exhibit cytotoxicity on microorganisms, there are a few reports on the promotive effects of Cs(+) on microbial growth under K(+)-deficient conditions. However, whether this growth-promoting effect is a common phenomenon remains uncertain, and direct correlation between growth promotion and Cs(+) uptake abilities has not been confirmed yet. Here, we validated the growth promotive effects of Cs(+) uptake under K(+)-deficient conditions using an Escherichia coli strain with an inducible expression of the Kup K(+) transporter that has nonspecific Cs(+) transport activities (strain kup-IE). The strain kup-IE exhibited superior growth under the Cs(+)-supplemented and K(+)-deficient conditions compared to the wild type and the kup null strains. The intracellular Cs(+) levels were significantly higher in strain kup-IE than in the other strains, and were well correlated with their growth yields. Furthermore, induction levels of the kup gene, intracellular Cs(+) concentrations, and the growth stimulation by Cs(+) also correlated positively. These results clearly demonstrated that Cs(+) incorporation via Kup transporter restores growth defects of E. coli under K(+)-deficient conditions.