Na+-stimulated nitrate uptake with increased activity under osmotic upshift in Synechocystis sp. strain PCC 6803

In the non-diazotrophic cyanobacterium Syn- echocystis sp. strain PCC 6803, an osmolality of 30 and 40 mosmol/kg sorbitol and NaCl resulted in 3.5- and 4.5-fold increase of nitrate uptake, respectively. The NaCl- stimulated uptake was abolished by treatment with chlor- amphenicol. At 25 mosmol/kg or higher, NaCl induced higher nitrate uptake than sorbitol suggesting an ionic effect of Na ? . The nitrate uptake in Synechocystis showed Ks and Vmax values of 46 lM and 1.37 lmol/min/mg Chl, respectively. Mutants disrupted in nitrate and nitrite reductase exhibited a decreased nitrate uptake. Ammo- nium, chlorate, and DL-glyceraldehyde caused a reduction of nitrate uptake. Dark treatment caused a drastic reduction of uptake by 70% suggesting an energy-dependent system. Nitrate transport was sensitive to various metabolic inhib- itors including those dissipating proton gradients and membrane potential. The results suggest that nitrate uptake in Synechocystis is stimulated by Na ? ions and requires energy provided by the functioning electron transport chain.