Characterization of Sulfate Transport in the Green Alga Chlorella ellipsoidea : ENVIRONMENTAL AND STRESS RESPONSES : MEMBRANES AND BIOENERGETICS

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A rapid induction of sulfate transport was observed in the green alga Chlorella ellipsoidea during sulfur-limited growth. Both affinity and V_<max> increased about five-fold within 6 h of transferring cells from Bold's basal medium with 350μM MgSO_4 to sulfur-deficient Bold's medium. High affinity sulfate transport was induced within 1 5 min and reached maximum rate within 3 h of transferring cells to sulfur-deficient condition, indicating that a new, high-affinity-sulfate transport system is induced by sulfur starvation in C. ellipsoidea. Eadie-Hofstee plots of initial rates of sulfate uptake indicated that the K_<1/2> Of sulfur-starved cells was about 17μM. Both sulfur-starved and unstarved cells grown in air had a V_<max> of 1.5 times higher than that of high-CO_2 grown cells. Sulfate transport was completely inhibited by 30μM CCCP or 800μM KCN both in the light and the dark but transport in the light was not inhibited by 20μM DCMU. Treatment with 50μM or 500μM vanadate caused 50% inhibition of uptake. The rate of sulfate up-take in the dark was twice that in the light and was stimulated by low pH. These results suggest that the sulfate transport system in C. ellipsoidea is operated by proton symport across the plasma-membrane which is partially mediated by P-type ATPase and that these systems depend exclusively on energy derived from oxidative phosphorylation in the mitochondria.
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