[Sulfate transport across the limiting double membrane or envelope, of spinach chloroplasts]

Evidence is presented for low rates of carrier-mediated uptake of sulphate, thiosulphate and sulphite into the stroma of the C3 plant Spinacia oleracea. Uptake of sulphate in the dark was followed using two techniques (1) uptake of sulphate [35S] as determined by silicon oil centrifugal filtration and (2) uptake as indicated by inhibition of CO2-dependent O2 evolution rates after addition of sulphate. Sulphate, thiosulphate and sulphite were transported across the envelope leading to an accumulation in the chloroplasts. Sulphate for each molecule of sulphate entering the chloroplast, one molecule of phosphate leaves the stroma, and vice-versa. The uptake of sulphate by isolated intact chloroplasts exchanging for internal free phosphate induced a lower rate of photophosphorylation, which in turn inhibited CO2-dependent O2 evolution. The presence, on the inner membrane of the chloroplasts envelope, of a specific sulphate carrier, distinct from the phosphate translocator, is discussed.