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-like putative Na+/K+ 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 ktr 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 muM), 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 DeltaktrB 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 DeltaktrB 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.