Comparative Analysis of kdp and ktr Mutants Reveals Distinct Roles of the Potassium Transporters in the Model Cyanobacterium

Photoautotrophic bacteria have developedmechanisms to maintain K homeostasis under conditions of changing ionic concen-trations in the environment. Synechocystis sp. strain PCC 6803 contains genes encoding a well-characterized Ktr-type K uptake transporter (Ktr) and a putative ATP-dependent transporter specific for K (Kdp). The contributions of each of these K trans-port systems to cellular K homeostasis have not yet been defined conclusively. To verify the functionality of Kdp, kdp genes were expressed in Escherichia coli, where Kdp conferred K uptake, albeit with lower rates than were conferred by Ktr. An on-chip microfluidic device enabled monitoring of the biphasic initial volume recovery of single Synechocystis cells after hyperos-motic shock. Here, Ktr functioned as the primary K uptake system during the first recovery phase, whereas Kdp did not con-tribute significantly. The expression of the kdp operon in Synechocystiswas induced by extracellular K depletion. Correspondingly, Kdp-mediated K uptake supported Synechocystis cell growth with trace amounts of external potassium. This induction of kdp expression depended on two adjacent genes, hik20 and rre19, encoding a putative two-component system. The circadian expression of kdp and ktr peaked at subjective dawn, which may support the acquisition of K required for the regular diurnal photosynthetic metabolism. These results indicate that Kdp contributes to the maintenance of a basal intracellular