Irihimovitch, V., Department of Life Sciences, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
Shapira, M., Department of Life Sciences, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel

Previous work showed a transient but dramatic arrest in the synthesis of Rubisco large subunit (LSU) upon transfer of Chlamydomonas reinhardtii cells from low light (LL) to high light (HL). Using dichlorofluorescin, a short- term 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 b6/f or by incubation with 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, ROS formation increased, and LSU synthesis ceased. The elevation of ROS occurred concurrently with a change in the redox state of the glutathione pool, which shifted toward its oxidized form immediately after the transfer to HL and returned to its original value after 6 h. The decrease in the reduced/oxidized glutathione ratio at HL was prevented by ascorbate and could be induced at LL by methyl viologen. We suggest that excess ROS mediate a decrease in the reduced/oxidized glutathione ratio that in turn signals the translational arrest of the rbcL transcript.