Glutathione redox potential modulated by reactive oxygen species regulatestranslation of Rubisco large subunit in the chloroplast

Previous work showed a transient but dramatic arrest in the synthesis of Ru bisco large subunit (LSU) upon transfer of Chlamydomonas reinhardtii cells from low light (LL) to high light (HL), Using dichlorofluorescin, a short-t erm increase in reactive oxygen species (ROS) was demonstrated, suggesting that their excessive formation could signal LSU down-regulation. A decrease in LSU synthesis occurred at LL in the presence of methyl viologen and was prevented at HL by ascorbate. Interfering with D1 function by mutations or by incubation with DCMU prevented the increase in ROS formation at HL and the concomitant down-regulation of LSU synthesis, If the electron transport was blocked further downstream, by mutation in the cytochrome b(6)/f or by incubation with 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, ROS forma tion increased, and LSU synthesis ceased. The elevation of ROS occurred con currently with a change in the redox state of the glutathione pool, which s hifted toward its oxidized form immediately after the transfer to HL and re turned to its original value after 6 h, The decrease in the reduced/oxidize d glutathione ratio at HL was prevented by ascorbate and could be induced a t LL by methyl viologen. We suggest that excess ROS mediate a decrease in t he reduced/oxidized glutathione ratio that in turn signals the translationa l arrest of the rbcL transcript.