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 HCO 3 ؊ 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 KtrABEmediated K ؉ uptake occurred with a moderately high affinity (K m of about 60 M), 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 ⌬ktrB 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 ⌬ktrB 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.
An Escherichia coli mutant deficient in genes for heme biosynthesis grew in medium of initial pH 8 containing 1% tryptone and glucose under aerobic growth conditions, and its doubling time was approximately 60 min at 37 degrees C. The growth rate was not increased under O2-limiting conditions. When the mutant was grown in medium of initial pH 6, growth stopped at the middle of the exponential growth phase. This could be overcome and the growth yield increased by the addition of 20 mM lysine to the growth medium. Lysine did not prevent the decrease in the medium pH as growth proceeded, making it unlikely that lysine decarboxylation stimulates growth by the alkalinization of the medium. These results indicate that respiration is not obligatory for growth under aerobic conditions, but growth without respiration at low pH requires a large amount of lysine.
An Escherichia coli mutant deficient in genes for heme biosynthesis grew in medium of initial pH 8 containing 1% tryptone and glucose under aerobic growth conditions, and its doubling time was approximately 60 min at 37 degrees C. The growth rate was not increased under O2-limiting conditions. When the mutant was grown in medium of initial pH 6, growth stopped at the middle of the exponential growth phase. This could be overcome and the growth yield increased by the addition of 20 mM lysine to the growth medium. Lysine did not prevent the decrease in the medium pH as growth proceeded, making it unlikely that lysine decarboxylation stimulates growth by the alkalinization of the medium. These results indicate that respiration is not obligatory for growth under aerobic conditions, but growth without respiration at low pH requires a large amount of lysine.
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