The cyanobacterium Synechococcus R‐2 (Anacystis nidulans, S. leopoliensis) PCC 7942 has a sodium‐dependent chloride transporter

Abstract

ABSTRACT Synechococcus R‐2 (PCC 7942) actively accumulated Cl− in the light and dark, under control conditions (BG‐11 media: pHo, 7·5; [Na+]o, 18 mol m−3; [Cl−]o, 0·508 molm−3). In BG‐11 medium [Cl−], was 17·2±0·848 mol m−3 (light), electrochemical potential of Cl− (ΔμCl−i,o) =+211±2mV; [Cl−]i= 1·24±0·11 mol m−3(dark), ΔμCl−i,o=+133±4mV. Cl− fluxes, but not permeabilities, were much higher in the light: ϕCl−i,o= 4·01±5·4 nmol m−2 s−1, PCl−i,o= 47±5pm s−1 (light); ϕCl−i,o= 0·395±0·071 nmol m−2 s−1, PCl−i,o= 69±14 pm s−1 (dark). Chloride fluxes are inhibited by acid pHo (pHo 5; ϕCl−i,o= 0·14±0·04 nmol m−2 s−1); optimal at pHo 7·5 and not strongly inhibited by alkaline pHo (pHo 10; ϕCl−1i,o= 1·7±0·14 nmol m−2 s−1). A Cl−in/2H+in coporter could not account for the accumulation of Cl− alkaline pHo. Permeability of Cl− is very low, below 100pm s−1 under all conditions used, and appears to be maximal at pHo 7·5 (50–70 pm s−1) and minimal in acid pHo (20pm s−1). DCCD (dicyclohexyl‐carbodiimide) inhibited ϕCl−i,o in the light about 75% and [Cl−]i fell to 2·2±0·26 (4) mol m−3. Valinomycin had no effect but monensin severely inhibited Cl− uptake ([Cl−]i= 1·02±0·32 mol m−3; ϕCl−i,o= 0·20±0·1 nmol m−2 s−1). Vanadate (200 mmol m−3) accelerated the Cl− flux (ϕCl−i,o= 5·28±0·64 nmol m−2 s−1) but slightly decreased accumulation of Cl− ([Cl−], = 13·9±1·3 mol m−3) in BG‐11 medium but had no significant effect in Na+‐free media. DCMU (dichlorophenyldimethylurea) did not reduce [Cl−], or ϕCl−i,o to that found in the dark ([Cl−]i= 8·41±0·76 mol m−3; ϕCl−i,o= 2·06±0·36 nmol m−2 s−1). Synechococcus also actively accumulated Cl− in Na+‐free media, [Cl−]i was lower but ΔΨi,o hyperpolarized in Na+‐free media and so the ΔμCl−i,o was little changed ([Cl−]i= 7·98±0·698 mol m−3; ΔμCl−i,o=+203±3 mV). Net Cl− uptake was stimulated by Na+; Li+ acted as a partial analogue for Na+. Synechococcus has a Na+ activated Cl− transporter which is probably a primary 2Cl−/ATP pump. The Cl− pump is voltage sensitive. ΔμCl−i,o is directly proportional to ΔΨi,o(P»0·01%): ΔμCl−i,o= ‐1·487 (±0·102) ×ΔΨi,o, r= ‐0·983, n= 31. The ΔμCl−i,o increased (more positive) as the Δμi,o became more negative. The ΔμCl−i,o has no known function, but might provide a driving force for the uptake of micronutrients.