Poster: Photosynthesis

Abs # 372: Intrathylakoid light stress signal transduction involves pH sensing by PsbS

Presenter:	Li, Xiao-Ping , xpli@uclink.berkeley.edu
Authors	Li, Xiao-Ping  (A)   Gilmore, Adam M (B)   Caffarri, Stefano  (C)   Bassi, Roberto  (C)   Golan, Talila  (A)   Kramer, David  (D)   NIyogi, Krishna K (A)
Affiliations:	(A): Department of Plant and Microbial Biology, University Of California (B): Research School of Biological Sciences, Institute of Advanced Studies, Australian National University (C): Department of Biology, University of Aix-Marseille II (D): Institute of Biological Chemistry and Department of Biochemistry and Biophysics, Washington State University

Feedback de-excitation (qE) regulates photosynthetic light harvesting by thermal dissipation of excess absorbed light energy. qE can be measured by the decrease or quenching of chlorophyll fluorescence due to reduced chlorophyll fluorescence lifetimes. A pH gradient across the thylakoid membrane is necessary for both qE, and a leaf absorption change at 535 nm (DA535) that is associated with qE. The DA535 has been attributed to a red shift in the absorption spectrum of zeaxanthin upon binding to PsbS, a photosystem II subunit. DCCD, a known inhibitor of qE, binds to PsbS. Site-directed mutagenesis, shows that two lumen-exposed glutamate residues (E122 and E226) in PsbS are necessary for qE in vivo. Mutations affecting either one or both glutamates showed they did not affect the xanthophyll cycle or the localization of PsbS in thylakoid membranes. However, the DA535, DCCD binding, and chlorophyll fluorescence lifetimes were affected, in addition to qE. The DA535 was decreased in all mutants in proportion to qE. PsbS in wild-type thylakoids did not bind DCCD at pH 7.8 but showed strong DCCD binding at pH 5. The E122Q and E226Q single mutants exhibited only 50% of the wild-type level of DCCD binding, while the E122Q E226Q double mutant lacked all DCCD binding. The E122Q E226Q double mutant had similar fluorescence lifetime distributions as a PsbS deletion mutant, whereas the single mutants had similar lifetime distribution patterns to the wild type but with smaller amplitudes of the faster lifetime components. These results indicate that protonation of E122 and E226 in excessive light induces a conformational change that activates the zeaxanthin-binding site(s) in PsbS. Upon binding to PsbS, zeaxanthin dissipates excess excitation energy in photosystem II as heat.