Sulfate uptake in photosynthetic Euglena gracilis. Mechanisms of regulation and contribution to cysteine homeostasis

Background

Sulfate uptake was analyzed in photosynthetic Euglena gracilis grown in sulfate sufficient or sulfate deficient media, or under Cd²⁺ exposure or Cys overload, to determine its regulatory mechanisms and contribution to Cys homeostasis.

Results

In control and sulfate deficient or Cd²⁺-stressed cells, one high affinity and two low affinity sulfate transporters were revealed, which were partially inhibited by photophosphorylation and oxidative phosphorylation inhibitors and ionophores, as well as by chromate and molybdate; H⁺ efflux also diminished in presence of sulfate. In both sulfate deficient and Cd²⁺-exposed cells, the activity of the sulfate transporters was significantly increased. However, the content of thiol-metabolites was lower in sulfate-deficient cells, and higher in Cd²⁺-exposed cells, in comparison to control cells. In cells incubated with external Cys, sulfate uptake was strongly inhibited correlating with 5-times increased intracellular Cys. Re-supply of sulfate to sulfate deficient cells increased the Cys, γ-glutamylcysteine and GSH pools, and to Cys-overloaded cells resulted in the consumption of previously accumulated Cys. In contrast, in Cd²⁺ exposed cells none of the already elevated thiol-metabolites changed.

Conclusions

(i) Sulfate transport is an energy-dependent process; (ii) sulfate transporters are over-expressed under sulfate deficiency or Cd²⁺ stress and their activity can be inhibited by high internal Cys; and (iii) sulfate uptake exerts homeostatic control of the Cys pool.