QUINONE BINDING SITES ON CYTOCHOROME b/c COMPLEXES*

332

225

Section: Identification Of Stm6 As a State Transitions Mutant Locked supporting

confidence: 57%

Improved Photobiological H2 Production in Engineered Green Algal Cells

Kruse

Rupprecht

Bader

et al. 2005

332

225

“…After the light was switched off, a reduction was observed that brought the signal to its initial level. In State 1 and State 2 conditions, the extent of the oxidation signal was equally sensitive to the addition of DBMIB, an inhibitor of cytochrome f reduction by plastoquinol (31) (Figs. 3, A and B, compare squares and triangles).…”

Section: Resultsmentioning

confidence: 99%

Photoinhibition of Chlamydomonas reinhardtii in State 1 and State 2

Finazzi¹,

Barbagallo²,

Bergo

et al. 2001

The relationship between state transitions and photoinhibition has been studied in Chlamydomonas reinhardtii cells. In State 2, photosystem II activity was more inhibited by light than in State 1. In State 2, however, the D1 subunit was not degraded, whereas a substantial degradation was observed in State 1. These results suggest that photoinhibition occurs via the generation of an intermediate state in which photosystem II is inactive but the D1 protein is still intact. The accumulation of this state is enhanced in State 2, because in this State only cyclic photosynthetic electron transport is active, whereas there is no electron flow between photosystem II and the cytochrome b 6 f complex (Finazzi, G., Furia, A., Barbagallo, R. P., and Forti, G. (1999) Biochim. Biophys. Acta 1413, 117-129). The activity of photosystem I and of cytochrome b 6 f as well as the coupling of thylakoid membranes was not affected by illumination under the same conditions. This allows repairing the damages to photosystem II thanks to cell capacity to maintain a high rate of ATP synthesis (via photosystem I-driven cyclic electron flow). This capacity might represent an important physiological tool in protecting the photosynthetic apparatus from excess of light as well as from other a-biotic stress conditions. The photochemical utilization of absorbed light is a critical step in the photosynthetic process. Because harvesting of light, photochemistry, and electron transfer occur on widely different scales of time, a correct balance among these different processes is required to optimize the efficiency of CO 2 fixation.When light is absorbed in excess of what can actually be utilized by photochemistry, damage to the photosynthetic apparatus may be induced. Impairment of both photosystem I (PSI) 1 (1) and photosystem II (PSII) (2) has been described, and this loss of activity has been termed photoinhibition (3). It has been also shown that the degradation of the PSII reaction center D1 subunit is a major consequence of photoinhibition (2).Some mechanisms contribute to protecting the photosynthetic apparatus from an excess of light (4, 5). The first is the so-called energy-dependent quenching, qE, i.e. the increased thermal dissipation in the PSII antennae that follows the generation of the electrochemical proton gradient across the thylakoid membranes. It is supposed to protect the reaction center from the consequences of a strong illumination by reducing the amount of energy present in the antenna protein complexes (6).The second one (6) is state transitions, a phenomenon that has been discovered in Chlorella pyrenoidosa (7) and in Porphyridium cruentum (8). It is a mechanism to balance light utilization between the two photosystems that is based on the reversible transfer of a fraction of the light-harvesting complex II (LHCII) from PSII to PSI (reviewed in Refs 9 -11). It is also supposed to protect PSII from photoinhibition inasmuch as it can decrease the size of its antenna.The migration of LHCII to PSI (State 1-State 2 transition) re...

“…Possible involvement of subunit IV (petD) of the cytochrome bf complex in close connection with the Rieske protein should also be considered (for review see Ref. 54).…”

Section: Discussionmentioning

confidence: 99%

Activation/Deactivation Cycle of Redox-controlled Thylakoid Protein Phosphorylation

Vener

Kan

Gal

et al. 1995

109

Signal transduction via light-dependent redox control of reversible thylakoid protein phosphorylation has evolved in plants as a unique mechanism for controlling events related to light energy utilization. Here we report for the first time that protein phosphorylation can be activated without light or the addition of reducing agents by a transient exposure of isolated thylakoid membranes to low pH in darkness. The activation of the kinase after incubation of dark-adapted thylakoids at pH 4.3 coincides with an increase in the plastoquinol: plastoquinone ratio up to 0.25. However, rapid plastoquinol reoxidation (<1 min) at pH 7.4 contrasts with the slow kinase deactivation (t1 ⁄2 ‫؍‬ 4 min), which indicates that the redox control is not directly dependent on the plastoquinone pool. Use of inhibitors and a cytochrome bf-deficient mutant of Lemna demonstrate the involvement of the cytochrome bf complex in the low-pH induced protein phosphorylation. EPR spectroscopy shows that subsequent to the transient low pH treatment and transfer of the thylakoids to pH 7.4, cytochrome f, the Rieske Fe-S center, and plastocyanin become reduced and are not reoxidized while the kinase is slowly deactivated. However, the deactivation correlates with a decrease of the EPR g z signal of the reduced Rieske Fe-S center, which is also affected by quinone analogues that inhibit the kinase. Our data point to an activation mechanism of thylakoid protein phosphorylation that involves the binding of plastoquinol to the cytochrome bf complex in the vicinity of the reduced Rieske Fe-S center.Protein phosphorylation plays a major role in cellular signaling, developmental processes, and metabolism regulation of living cells (1-3). In chloroplasts a unique light mediated redox-controlled phosphorylation (4 -7) of a number of proteins associated with the thylakoid membrane has evolved. Phosphorylation of the major light-harvesting chlorophyll a/b protein complex (LHCII) 1 regulates the balance of excitation energy between the two photosystems (5, 6, 8), protects oxygen-evolving organisms against photoinhibition by excessive light excitation (9) and may affect the process of LHCII degradation related to the long term acclimation of the light-harvesting antenna size to the prevailing ambient light intensity (10). Other identified phosphoproteins belong to photosystem II and include the D1 and D2 reaction center protein subunits as well as the chlorophyll a binding protein CP43 and the 9-kDa psbH protein (11,12). Phosphorylation of the D1 polypeptide in higher plants is implicated in the regulation of its degradation during the light induced turnover and repair of photoinhibitory damage to the photosystem II reaction center (13-16).The specific mechanism involved in the redox-mediated activation of the thylakoid kinase(s) is not yet understood. Activation of thylakoid protein phosphorylation is dependent on the redox state of the plastoquinone pool (5-7). In cytochrome bf-deficient mutants of algae (17) and higher plants (18 -20) the redox-controlle...