Na super(+)-dependent K super(+) 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 super(+)/K super(+) transporter gene in the cyanobacterium Synechocystis sp. PCC 6803 confers increased Na super(+) sensitivity, and inhibits [Formula: see text] 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 super(+) transport activity. They encode an NAD super(+)-binding peripheral membrane protein (ktrA; sll0493), an integral membrane protein, belonging to a superfamily of K super(+) 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 super(+) uptake occurred with a moderately high affinity (K sub(m) of about 60 mu M), and depended on both Na super(+) and a high membrane potential, but not on ATP. KtrABE neither mediated Na super(+) uptake nor Na super(+) efflux. In Synechocystis sp. PCC 6803, KtrB-mediated K super(+) uptake required Na super(+) 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 super(+) from the cells. The deltaktrB cells shocked with sorbitol failed to reaccumulate K super(+) up to its original level. These data indicate that in strain PCC 6803 K super(+) uptake via KtrABE plays a crucial role in the early phase of cell turgor regulation after hyperosmotic shock.