f Photoautotrophic bacteria have developed mechanisms to maintain K ؉ homeostasis under conditions of changing ionic concentrations 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 ؉ transport 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 onchip microfluidic device enabled monitoring of the biphasic initial volume recovery of single Synechocystis cells after hyperosmotic shock. Here, Ktr functioned as the primary K ؉ uptake system during the first recovery phase, whereas Kdp did not contribute significantly. The expression of the kdp operon in Synechocystis was 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 K Living cells have developed specific responses to hyperosmotic shock. Upon exposure to this stress, cells initially lose water and their volume shrinks. In all living cells, K ϩ is the major intracellular cation used for the maintenance of turgor pressure, cytosolic osmolarity, protein structuring, and membrane potential (1-3). In contrast to animals, Na ϩ /K ϩ ATP pumps are generally missing in bacteria and plants. Hence, these cells possess K ϩ uptake transporters to supply K ϩ to the cells. Particularly after hyperosmotic stress, cells quickly take up K ϩ from the medium to increase the intracellular osmolarity, which prevents water efflux from the cell. Data from genetic and biochemical experiments indicate that the activity and the expression of these transporters respond to hyperosmotic stress. In the later phase of acclimation to hyperosmotic stress, cells also induce the synthesis of osmoprotective molecules, such as glutamate, trehalose, proline, and glucosylglycerol (4, 5). Despite an increasing amount of data on cellular osmoregulation involving ion flux across the membrane, direct evidence for the involvement of specific transporters in the cellular response to osmotic up-shock is lacking for photoautotrophic organisms.The cyanobacterium Synechocystis sp. strain PCC 6803 (hereinafter referred to as Synechocystis) is a frequently used unicellular photosynthetic prokaryote that can survive under a wide range of environmental conditions (6). Unlike Escherichia coli, Synechocystis possesses an internal thylakoid membrane system, which...