Role of a novel photosystem II-associated carbonic anhydrase in photosynthetic carbon assimilation in Chlamydomonas reinhardtii.
(1/1996)
Intracellular carbonic anhydrases (CA) in aquatic photosynthetic organisms are involved in the CO2-concentrating mechanism (CCM), which helps to overcome CO2 limitation in the environment. In the green alga Chlamydomonas reinhardtii, this CCM is initiated and maintained by the pH gradient created across the chloroplast thylakoid membranes by photosystem (PS) II-mediated electron transport. We show here that photosynthesis is stimulated by a novel, intracellular alpha-CA bound to the chloroplast thylakoids. It is associated with PSII on the lumenal side of the thylakoid membranes. We demonstrate that PSII in association with this lumenal CA operates to provide an ample flux of CO2 for carboxylation. (+info)
Lipophilicity determination of some potential photosystem II inhibitors on reversed-phase high-performance thin-layer chromatography.
(2/1996)
The retention characteristics of 25 2-cyano-3-methylthio-3-substituted amine-acrylates are determined using reversed-phase thin-layer chromatography (RP-TLC) with methanol-water mixtures as eluents. The relationship between Rm values and partition coefficients (C log P) are established. The Rm values decrease linearly with increasing methanol concentration in the eluent. The Rm values extrapolated to zero organic modifier concentration (Rm0) in the eluent are highly related to C log P. The Rm0 value can be used to evaluate the lipophilicity of this kind of compound. (+info)
A functional model for O-O bond formation by the O2-evolving complex in photosystem II.
(3/1996)
The formation of molecular oxygen from water in photosynthesis is catalyzed by photosystem II at an active site containing four manganese ions that are arranged in di-mu-oxo dimanganese units (where mu is a bridging mode). The complex [H2O(terpy)Mn(O)2Mn(terpy)OH2](NO3)3 (terpy is 2,2':6', 2"-terpyridine), which was synthesized and structurally characterized, contains a di-mu-oxo manganese dimer and catalyzes the conversion of sodium hypochlorite to molecular oxygen. Oxygen-18 isotope labeling showed that water is the source of the oxygen atoms in the molecular oxygen evolved, and so this system is a functional model for photosynthetic water oxidation. (+info)
D1-D2 protein degradation in the chloroplast. Complex light saturation kinetics.
(4/1996)
The D1 and D2 proteins of the photosystem II (PSII) reaction center are stable in the dark, while rapid degradation occurs in the light. Thus far, a quantitative correlation between degradation and photon fluences has not been determined. In Spirodela oligorrhiza, D1-D2 degradation increases with photon flux. We find that kinetics for D2 degradation mirror those for D1, except that the actual half-life times of the D2 protein are about three times larger than those of the D1. The degradation ratio, D2/D1, is fluence independent, supporting the proposal [Jansen, M.A.K., Greenberg, B.M., Edelman, M., Mattoo, A.K. & Gaba, V. (1996), Photochem. Photobiol. 63, 814-817] that degradation of the two proteins is coupled. It is commonly conceived that D1 degradation is predominantly associated with photon fluences that are supersaturating for photosynthesis. We now show that a fluence as low as 5 mumol.m-2.s-1 elicited a reaction constituting > 25% of the total degradation response, while > 90% of the degradation potential was attained at intensities below saturation for photosynthesis (approximately 750 mumol.m-2.s-1). Thus, in intact plants, D1 degradation is overwhelmingly associated with fluences limiting for photosynthesis. D1 degradation increases with photon flux in a complex, multiphasic manner. Four phases were uncovered over the fluence range from 0-1600 mumol.m-2.s-1. The multiphasic saturation kinetics underscore that the D1 and D2 degradation response is complex, and emanates from more than one parameter. The physiological processes associated with each phase remain to be determined. (+info)
Construction and characterization of a functional mutant of Synechocystis 6803 harbouring a eukaryotic PSII-H subunit.
(5/1996)
A Synechocystis 6803 mutant carrying a chimaeric photosystem II (PSII), in which the Zea mays PsbH subunit (7.7 kDa calculated molecular mass) replaces the cyanobacterial copy (7.0 kDa), was constructed. With the exception of the N-terminal 12 amino acid extension, which has a phosphorylatable threonine, the eukaryotic polypeptide is 78% homologous to its bacterial counterpart. Biochemical characterization of this mutant shows that it expresses the engineered gene correctly and is competent for photoautotrophic growth. Fluorescence analysis and oxygen evolution measurements in the presence of exogenous acceptors indicate that the observed phenotype results from a chimaeric PSII rather than from the absence of function associated with PsbH, suggesting that the heterologous protein is assembled into a functional PSII. Inhibition of oxygen evolution by herbicides belonging to different classes shows that the sensitivity of the mutant PSII is changed only towards phenolic compounds. This result indicates slight conformational modification of the QB/herbicide binding pocket of the D1 polypeptide caused by the bulky PsbH protein in the mutant, and also suggests close structural interaction of the D1 and PsbH subunits in the topological arrangement of PSII. (+info)
Determination of the stoichiometry and strength of binding of xanthophylls to the photosystem II light harvesting complexes.
(6/1996)
Xanthophylls have a crucial role in the structure and function of the light harvesting complexes of photosystem II (LHCII) in plants. The binding of xanthophylls to LHCII has been investigated, particularly with respect to the xanthophyll cycle carotenoids violaxanthin and zeaxanthin. It was found that most of the violaxanthin pool was loosely bound to the major complex and could be removed by mild detergent treatment. Gentle solubilization of photosystem II particles and thylakoids allowed the isolation of complexes, including a newly described oligomeric preparation, enriched in trimers, that retained all of the in vivo violaxanthin pool. It was estimated that each LHCII monomer can bind at least one violaxanthin. The extent to which different pigments can be removed from LHCII indicated that the relative strength of binding was chlorophyll b > neoxanthin > chlorophyll a > lutein > zeaxanthin > violaxanthin. The xanthophyll binding sites are of two types: internal sites binding lutein and peripheral sites binding neoxanthin and violaxanthin. In CP29, a minor LHCII, both a lutein site and the neoxanthin site can be occupied by violaxanthin. Upon activation of the violaxanthin de-epoxidase, the highest de-epoxidation state was found for the main LHCII component and the lowest for CP29, suggesting that only violaxanthin loosely bound to LHCII is available for de-epoxidation. (+info)
Protective function of chloroplast 2-cysteine peroxiredoxin in photosynthesis. Evidence from transgenic Arabidopsis.
(7/1996)
2-Cysteine peroxiredoxins (2-CPs) constitute a ubiquitous group of peroxidases that reduce cell-toxic alkyl hydroperoxides to their corresponding alcohols. Recently, we cloned 2-CP cDNAs from plants and characterized them as chloroplast proteins. To elucidate the physiological function of the 2-CP in plant metabolism, we generated antisense mutants in Arabidopsis. In the mutant lines a 2-CP deficiency developed during early leaf and plant development and eventually the protein accumulated to wild-type levels. In young mutants with reduced amounts of 2-CP, photosynthesis was impaired and the levels of D1 protein, the light-harvesting protein complex associated with photosystem II, chloroplast ATP synthase, and ribulose-1,5-bisphosphate carboxylase/oxygenase were decreased. Photoinhibition was particularly pronounced after the application of the protein synthesis inhibitor, lincomycin. We concluded that the photosynthetic machinery needs high levels of 2-CP during leaf development to protect it from oxidative damage and that the damage is reduced by the accumulation of 2-CP protein, by the de novo synthesis and replacement of damaged proteins, and by the induction of other antioxidant defenses in 2-CP mutants. (+info)
Systemic signaling and acclimation in response to excess excitation energy in Arabidopsis.
(8/1996)
Land plants are sessile and have developed sophisticated mechanisms that allow for both immediate and acclimatory responses to changing environments. Partial exposure of low light-adapted Arabidopsis plants to excess light results in a systemic acclimation to excess excitation energy and consequent photooxidative stress in unexposed leaves. Thus, plants possess a mechanism to communicate excess excitation energy systemically, allowing them to mount a defense against further episodes of such stress. Systemic redox changes in the proximity of photosystem II, hydrogen peroxide, and the induction of antioxidant defenses are key determinants of this mechanism of systemic acquired acclimation. (+info)