Sulfate uptake chloroplasts

The uptake of sulfate by chloroplasts in exchange for phosphate, triose phosphate, or one of the other substrates transported by the phosphate translocator affords an explanation for the effect of sulfate on COj assimilation by isolated chloroplasts. In the presence of CO2, O2 evolution is completely inhibited by 20 mM sulfate (Baldry et ai, 1968). Addition of phosphate alleviates the sulfate inhibition. In the absence of sulfate, phosphate also inhibits O2 evolution. The latter is attributed to efflux of triose phosphate in exchange for phosphate, thereby preventing the reformation of ribulose 1,5-bisphosphate from triose phosphate for the continued assimilation ofCOj (Edwards and Walker, 1983). Presumably sulfate inhibits O2 evolution for similar reasons and alleviation of the sulfate inhibition by phosphate involves sulfate/phosphate exchange, thereby permitting triose phosphate to be retained in the chloroplast for the synthesis of ribulose 1,5-bisphosphate and hence the resumption of O2 evolution. 

---