Minisymposium 2: Oxidative Stress
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M0203: Photosystem-II damage and repair cycle in chloroplasts
An irreversible photooxidative damage occurs frequently in PSII of all chloroplasts, especially under environmental stress conditions, limiting photosynthesis and causing losses in plant growth and productivity. The repair process rectifies this adverse effect by selectively removing and replacing the photoinactivated D1/32 kD reaction center protein (the chloroplast-encoded psbA gene product) from the massive (>1,000 kD) H2O-oxidizing and O2-evolving PSII holocomplex. This repair is unique in the annals of biology; nothing analogous in complexity and specificity has been reported in other biological systems. The research employs genetic, molecular and biochemical approaches to identify genes and proteins of the PSII repair process. It may reveal hitherto unknown reactions for the selective in situ replacement of single proteins from within multi-protein complexes. Current advances include: (a) Application of DNA insertional mutagenesis in the model green alga Chlamydomonas reinhardtii resulting in the isolation of five putative PSII repair mutants (rep5, rep9, rep16, rep27 and rep55). (b) Work on strain rep55 revealed a novel nuclear-encoded chloroplast sulfate permease, which regulates the uptake of sulfate and the PSII repair process in chloroplasts. (c) Research on the configuration of PSII repair intermediates and events following photodamage resulted in the identification of HSP70B and ELIP(Cbr)-type proteins, as well as zeaxanthin that accumulate in thylakoids along with photodamaged PSII reaction centers. These may form a distinct PSII repair supercomplex. (d) Molecular and biochemical analysis of rep5 showed plasmid insertion in the promoter region of a putative CSN6 gene in this strain, potentially impairing transcription and/or translation of the CSN6 protein. The research addresses the role of the csn6 mutation in the PSII repair-aberrant phenotype. (e) Molecular analysis of rep16 and rep27 showed that ORFs encoding for proteins of unknown function have been deleted upon plasmid insertion in these mutants. The research characterizes the molecular properties of the deleted ORFs and elucidates roles of the respective genes in the PSII repair process. In summary, the work generates fundamental knowledge of genes, proteins, and networks involved in the response of all plants to irradiance stress, especially when it is applied in combination with other environmental stresses, e.g. water, temperature or CO2 limitation.
Supported by a USDA-NRI grant.
