Poster: Oxidative stress

Abs # 69: Structural alterations in Rubisco subunits during photo-oxidative stress interrupts Rubisco assembly

Presenter:	Cohen, Idan , cohenida@bgumail.bgu.ac.il
Authors	Cohen, Idan  (A)   Irihimovitch, Vered  (A)   Knopf, Yoel  (A)   Shapira, Michal  (A)
Affiliations:	(A): Ben Gurion university

Ribulose biphosphate carboxylase (Rubisco) is the key enzyme in carbon assimilation during photosynthesis. In green algae and in land plants the enzyme is composed of eight large subunits (LSU) encoded by the chloroplast rbcL gene and eight small subunits (SSU) encoded by the nuclear rbcS gene family. Transfer of Chlamydomonas cells from low-light (LL) to high-light (HL) causes an increase in reactive oxygen radicals (ROS) and in the oxidation of the glutathione pool. We previously observed a dramatic arrest in LSU translation that correlates with the changes in the intra-cellular redox state of the glutathione pool. An oxidizing environment causes conformational changes in both small and large subunits in vitro, in the presence of GSSG. Similar changes were observed in vivo, in response to the light stress that occurs during transfer from LL to HL. The redox state of the thiol groups was monitored by the thiol reacting agent AMS. Assembly of Rubisco holoenzyme under light-induced oxidative stress is interrupted, as shown on native gels. Different mechanisms are employed by the algal cell, to prevent the accumulation of non- assembled Rubisco subunits during light stress. We show that translation of the LSU is arrested while the SSU is rapidly degraded under these conditions. We propose that the structural changes serve as a physiological sensor for assembly. Recently we showed that under oxidating conditions, the amino terminus of Rubisco LSU is capable of binding to RNA and resembles in its structure the RNA Binding Domain (RBD) of known RNA binding proteins. Under reducing conditions, this domain is buried in the protein, and is exposed only upon oxidation. This domain could be involved in an autoregulatory loop that controls LSU translation.