Na⁺-dependent K⁺ uptake Ktr system from the cyanobacterium Synechocystis sp. PCC 6803 and its role in the early phases of cell adaptation to hyperosmotic shock

Transmembrane ion transport processes play a key role in the adaptation of cells to hyperosmotic conditions. Previous work has shown that the disruption of a ktrB/ntpJ-Wse putative Na⁺TK⁺ transporter gene in the cyanobacterium Synechocystis sp. PCC 6803 confers increased Na⁺ sensitivity, and inhibits HCO₃^- uptake. Here, we report on the mechanistic basis of this effect. Heterologous expression experiments in Escherichia coli show that three Synechocystis genes are required for K ⁺ transport activity. They encode an NAD⁺-binding peripheral membrane protein (ktrA; sll0493), an integral membrane protein, belonging to a superfamily of K⁺ transporters (ktrB; formerly ntpJ; slr1509), and a novel type of kfr gene product, not previously found in Ktr systems (ktrE; slr1508). In E. coli, Synechocystis KtrABE-mediated K⁺ uptake occurred with a moderately high affinity (K_m of about 60 μM), and depended on both Na⁺ and a high membrane potential, but not on ATP. KtrABE neither mediated Na⁺ uptake nor Na⁺ efflux. In Synechocystis sp. PCC 6803, KtrB-mediated K⁺ uptake required Na⁺ and was inhibited by protonophore. A ΔktrB strain was sensitive to long term hyperosmotic stress elicited by either NaCl or sorbitol. Hyperosmotic shock led initially to loss of net K⁺ from the cells. The ΔktrB cells shocked with sorbitol failed to reaccumulate K⁺ up to its original level. These data indicate that in strain PCC 6803 K⁺ uptake via KtrABE plays a crucial role in the early phase of cell turgor regulation after hyperosmotic shock.