Specific 125 I labeling of D1 (herbicide-binding protein)

An enzyme kinetics study of the pH dependence of chloride activation of oxygen evolution in photosystem II

Oxygen evolution by photosystem II (PSII) involves activation by Cl^- ion, which is regulated by extrinsic subunits PsbQ and PsbP. In this study, the kinetics of chloride activation of oxygen evolution was studied in preparations of PSII depleted of the PsbQ and PsbP subunits (NaCl-washed and Na₂SO₄/pH 7.5-treated) over a pH range from 5.3 to 8.0. At low pH, activation by chloride was followed by inhibition at chloride concentrations >100 mM, whereas at high pH activation continued as the chloride concentration increased above 100 mM. Both activation and inhibition were more pronounced at lower pH, indicating that Cl^- binding depended on protonation events in each case. The simplest kinetic model that could account for the complete data set included binding of Cl^- at two sites, one for activation and one for inhibition, and four protonation steps. The intrinsic (pH-independent) dissociation constant for Cl^- activation, K _S, was found to be 0.9 ± 0.2 mM for both preparations, and three of the four pK _as were determined, with the fourth falling below the pH range studied. The intrinsic inhibition constant, K _I, was found to be 64 ± 2 and 103 ± 7 mM for the NaCl-washed and Na₂SO₄/pH7.5-treated preparations, respectively, and is considered in terms of the conditions likely to be present in the thylakoid lumen. This enzyme kinetics analysis provides a more complete characterization of chloride and pH dependence of O₂ evolution activity than has been previously presented.

Carbohydrate metabolism in mutants of the cyanobacterium Synechococcus elongatus PCC 7942 defective in glycogen synthesis

ADP-glucose pyrophosphorylase (AGPase) and glycogen synthase (GS) catalyze the first two reactions of glycogen synthesis in cyanobacteria. Mutants defective in each of these enzymes in Synechococcus elongatus PCC 7942 were constructed and characterized. Activities of the corresponding enzymes in the selected mutants were virtually undetectable, and their ability to synthesize glycogen was entirely abolished. The maximal activities of photosynthetic O(2) evolution and the rates of respiration in the dark were significantly decreased in the mutants compared to those in wild-type cells. Addition of 0.2 M NaCl or 3 mM H(2)O(2) to liquid cultures markedly inhibited the growth of the AGPase and GS mutants, while the same treatment had only marginal effects on the wild type. These results suggest a significant role for storage polysaccharides in tolerance to salt or oxidative stress.

Characteristics of Iodide Activation and Inhibition of Oxygen Evolution by Photosystem II

Oxygen evolution by photosystem II (PSII) is activated by chloride and other monovalent anions. In this study, the effects of iodide on oxygen evolution activity were investigated using PSII-enriched membrane fragments from spinach. In the absence of Cl(-), the dependence of oxygen evolution activity on I(-) concentration showed activation followed by inhibition in both intact PSII and NaCl-washed PSII, which lacked the PsbP and PsbQ subunits. Using a substrate inhibition model, the range of values of the Michaelis constant K(M) in intact PSII (0.5-1.5 mM) was smaller than that in NaCl-washed PSII (1.5-5 mM), whereas values of the inhibition constant K(I) in intact PSII (9-17 mM) were larger than those in NaCl-washed PSII (1-4 mM). Studies of I(-) inhibition of Cl(-)-activated oxygen evolution in intact PSII revealed that I(-) was primarily an uncompetitive inhibitor, with uncompetitive constant K(i)' = 37 mM and Cl(-)-competitive constant K(i) > 200 mM. This result indicated that the activating Cl(-) must be bound for inhibition to take place, which is consistent with the substrate inhibition model for I(-) activation. The S(2) state multiline and g = 4.1 EPR signals in NaCl-washed PSII were examined in the presence of 3 and 25 mM NaI, corresponding to I(-)-activated and I(-)-inhibited conditions, respectively. The two S(2) state signals were observed at both I(-) concentrations, indicating that I(-) substitutes for Cl(-) in formation of the signals and that advancement to the S(2) state was not prevented by high I(-) concentrations. A model is presented that incorporates the results of this study, including the action of both chloride and iodide.

Differing involvement of sulfoquinovosyl diacylglycerol in photosystem II in two species of unicellular cyanobacteria

Sulfoquinovosyl diacylglycerol (SQDG) is involved in the maintenance of photosystem II (PSII) activity in Chlamydomonas reinhardtii[Minoda, A., Sato, N., Nozaki, H., Okada, K., Takahashi, H., Sonoike, K. & Tsuzuki, M. et al. (2002) Eur. J. Biochem.269, 2353-2358]. To understand the spread of the taxa in which PSII interacts with SQDG, especially in cyanobacteria, we produced a mutant defective in the putative sqdB gene responsible for SQDG synthesis from two cyanobacteria, Synechocystis sp. PCC6803 and Synechococcus sp. PCC7942. The mutant of PCC6803, designated SD1, lacked SQDG synthetic ability and required SQDG supplementation for its growth. After transfer from SQDG-supplemented to SQDG-free conditions, SD1 showed decreased net photosynthetic and PSII activities on a chlorophyll (Chl) basis with a decrease in the SQDG content. Moreover, the sensitivity of PSII activity to 3-(3,4-dichlorophenyl)-1,1-dimethylurea and atrazine was increased in SD1. However, SD1 maintained normal amounts of cytochrome b559 and D1 protein (the subunits comprising the PSII complex) on a Chl basis, indicating that the PSII complex content changed little, irrespective of a decrease in the SQDG content. These results suggest that the role of SQDG is the conservation of the PSII properties in PCC6803, consistent with the results obtained with C. reinhardtii. In contrast, the SQDG-null mutant of PCC7942 showed the normal level of PSII activity with little effect on its sensitivity to PSII herbicides. Therefore, the difference in the SQDG requirement for PSII is species-specific in cyanobacteria; this could be of use when investigating the molecular evolution of the PSII complex.

A novel gene, pmgA, specifically regulates photosystem stoichiometry in the cyanobacterium Synechocystis species PCC 6803 in response to high light

Previously, we identified a novel gene, pmgA, as an essential factor to support photomixotrophic growth of Synechocystis species PCC 6803 and reported that a strain in which pmgA was deleted grew better than the wild type under photoautotrophic conditions. To gain insight into the role of pmgA, we investigated the mutant phenotype of pmgA in detail. When low-light-grown (20 microE m(-2) s(-1)) cells were transferred to high light (HL [200 microE m(-2) s(-1)]), pmgA mutants failed to respond in the manner typically associated with Synechocystis. Specifically, mutants lost their ability to suppress accumulation of chlorophyll and photosystem I and, consequently, could not modulate photosystem stoichiometry. These phenotypes seem to result in enhanced rates of photosynthesis and growth during short-term exposure to HL. Moreover, mixed-culture experiments clearly demonstrated that loss of pmgA function was selected against during longer-term exposure to HL, suggesting that pmgA is involved in acquisition of resistance to HL stress. Finally, early induction of pmgA expression detected by reverse transcriptase-PCR upon the shift to HL led us to conclude that pmgA is the first gene identified, to our knowledge, as a specific regulatory factor for HL acclimation.

Redox dependence for photoligation of manganese to the apo-water-oxidizing complex in chloroplasts and photosystem II membranes

Effects of reducing reagents and redox potentials on photoactivation were studied in Mn-depleted chloroplasts and PSII membranes. Exogenous reducing reagents abolished photoactivation in PSII membranes, while they stimulated photoactivation in chloroplasts. To determine how reducing reagents can have such opposing effects in these preparations, we studied how redox potentials affect photoactivation in the range from 0 mV to +500 mV. In chloroplasts, a modest yield of photoactivation was obtained in the redox potential range of +100 and +330 mV at pH 7.5. The yield of photoactivation decreased at redox potentials above +330 mV, and drastically increased below potentials of +100 mV. Nernst plots of the data show that an n = 1 redox component with an Em7.5 of +374 mV, as well as an n = 2 redox component with an Em7.5 of +61 mV, is involved in photoactivation of chloroplasts isolated from dark-grown spruce seedlings. In the case of PSII membranes, photoactivation decreased sharply on either side of +335 mV at pH 5.5. The n = 1 redox components with Em5.5 of +375 and +319 mV may be involved, both of which showed pH dependences of -60 mV/pH unit. DCMU abolished photoactivation in chloroplasts, but did not affect the dependence of photoactivation on oxidation-reduction potentials in PSII membranes. The component with an Em5.5 of +319 mV involved in photoactivation of PSII membranes was also observed in the dependence of Mn solubilization on oxidation-reduction potentials with PSII membranes lacking extrinsic proteins, suggesting that the reduction of Mn with higher valences to Mn(II) by exogenous reducing reagents reversibly occurs in the intermediates or an active center during photoactivation in PSII membranes. Involvement of such redox components in photoactivation in chloroplasts and PSII membranes is discussed.

ATP-dependent protein synthesis in isolated pea chloroplasts. Evidence for accumulation of a translation intermediate of the D1 protein

In the presence of externally added ATP, in the dark, isolated pea chloroplasts accumulate two proteins of molecular masses of about 22 and 24 kDa which precipitate with specific antibodies raised against the D1 protein. By chasing in the light, these proteins disappeared on the fluorogram concomitant with the appearance of the precursor- and mature-sized D1 proteins. Polysome analysis indicated that the 22-kDa component is associated with membrane-bound ribosomes and is thus ascribed to a translation intermediate of the D1 protein. On the other hand, the 24-kDa component could not be found in the polysome fraction under the experimental condition used, suggesting the possibility that this component is a degradation product of the D1 protein. The conclusion from this analysis is that the synthesis and/or stable accumulation of the D1 protein requires factor(s) caused by illumination, in addition to ATP, in isolated pea chloroplasts.

The role of redox-active amino acids in the photosynthetic water-oxidizing complex

The protein environment can dramatically affect the EPR line shape of tyrosine radicals. The alterations can be caused by: (1) a change in methylene geometry caused by different protein steric constraints; (2) a change in spin density caused by a change in protein environment; or (3) covalent modification of the tyrosine. Any or all of these effects may also be important, in some cases, in control of oxidation potential and chemical reactivity. The new signal that has been observed in the YF161D1 PS II mutant has an approximate 1:3:3:1 lineshape. There is no precedent for a 1:3:3:1 EPR signal from a tyrosine in a powder sample. However, as described above, given the diversity of signals from tyrosine radicals, it is impossible to exclude the possibility that the signal arises from tyrosine on this basis.

Interactions of iodide ions with isolated photosystem 2 particles

The effects of I- ions on O2 evolution by photosystem 2 particles, which were depleted of the 18-kDa and the 23-kDa extrinsic proteins of the O2 evolution complex by NaCl washing (dPS2 particles) were examined. In the absence of Cl- (incompetent dPS2) I- stimulated O2 evolution up to 3-6 mM, depending on the associated cation, and inhibited it at higher concentrations. In the presence of Cl- (competent dPS2), I- was inhibitory at all concentrations. The inhibition was reversible, it occurred at a site preceding Tyrz (Tyr residue mediating electron transfer from H2O to photosystem 2), and it interfered noncompetitively with the reactivation of incompetent dPS2 with Cl-. Furthermore, the organic salts tetrabutyl ammonium iodide and tetraphenyl phosphonium iodide proved to be stronger inhibitors than the inorganic NaI. This is interpreted as an indication of a negatively charged surface, situated behind a hydrophobic permeability barrier. Permeant organic cations, being better compensators of the inner surface charge than Na+, are also more apt in facilitating access of the I- ions to the inhibitory site in the vicinity of Tyrz.

Current perceptions of Photosystem II

In the last few years our knowledge of the structure and function of Photosystem II in oxygen-evolving organisms has increased significantly. The biochemical isolation and characterization of essential protein components and the comparative analysis from purple photosynthetic bacteria (Deisenhofer, Epp, Miki, Huber and Michel (1984) J Mol Biol 180: 385-398) have led to a more concise picture of Photosystem II organization. Thus, it is now generally accepted that the so-called D1 and D2 intrinsic proteins bind the primary reactants and the reducing-side components. Simultaneously, the nature and reaction kinetics of the major electron transfer components have been further clarified. For example, the radicals giving rise to the different forms of EPR Signal II have recently been assigned to oxidized tyrosine residues on the D1 and D2 proteins, while the so-called Q400 component has been assigned to the ferric form of the acceptor-side iron. The primary charge-separation has been meaured to take place in about 3 ps. However, despite all recent major efforts, the location of the manganese ions and the water-oxidation mechanism still remain largely unknown. Other topics which lately have received much attention include the organization of Photosystem II in the thylakoid membrane and the role of lipids and ionic cofactors like bicarbonate, calcium and chloride. This article attempts to give an overall update in this rapidly expanding field.

Directed mutagenesis indicates that the donor to P+680 in photosystem II is tyrosine-161 of the D1 polypeptide

Photosystem II contains two redox-active tyrosines. One of these, YZ, reduces the reaction center chlorophyll, P680, and transfers the oxidizing equivalent to the oxygen-evolving complex. The second, YD, has a long-lived free radical state of unknown function. We recently established that YD is Tyr-160 of the D2 polypeptide by site-directed mutagenesis of a psbD gene in the unicellular cyanobacterium Synechocystis 6803 [Debus, R. J., Barry, B. A., Babcock, G. T., & McIntosh, L. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 427-430]. YZ is most likely the symmetry-related Tyr-161 of the D1 polypeptide. To test this hypothesis, we have changed Tyr-161 to phenylalanine by site-directed mutagenesis of a psbA gene in Synechocystis. The resulting mutant assembles PSII, as judged by its ability to produce the stable Y+D radical, but is unable to grow photosynthetically and exhibits altered fluorescence properties. The nature of the fluorescence change indicates that forward electron transfer to P+680 is disrupted in the mutant. These results provide strong support for our identification of Tyr-161 in the D1 polypeptide with YZ.

Purification and characterization of an iron stress-induced chlorophyll-protein from the cyanobacterium Anacystis nidulans R2

An Anacystis nidulans R2 chlorophyll-protein associated with Photosystem II in iron-stressed cells (Pakrasi, H.B., Riethmann, H.C. and Sherman, L.A. (1985) Proc. Natl. Acad. Sci. USA 82, 6903-6907) has been biochemically purified and characterized. Anion exchange chromatography of dodecyl-beta-D-maltoside-solubilized membranes from iron-deficient cells was used to recover this chlorophyll-protein (termed CPVI-4) in high yield and in a relatively native state. CPVI-4 has a room temperature absorption maximum at 671 nm, a 77 K chlorophyll fluorescence peak at 681 nm, and contains polypeptides of 36, 34 and 12 kDa. The 36 and 34 kDa polypeptides are associated with chlorophyll on mildly denaturing acrylamide gels of purified CPVI-4, although only the 34 kDa protein is immunoreactive with antisera elicited against the gel-purified chlorophyll-protein. Immunoblotting experiments with dodecyl-beta-D-maltoside-solubilized membrane fractions and purified CPVI-4 indicate that CPVI-4 does not contain previously identified Photosystem II core proteins. CPVI-4 likely functions as a light-harvesting antenna complex in iron-starved cells (where phycobilisomes are absent or diminished) and, in addition, may contribute chlorophyll to the reaction center complexes during their assembly in the early stages of recovery from iron stress.

Multiple action sites for photosystem II herbicides as revealed by delayed fluorescence

DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) at concentrations higher than 10 μM suppresses the second time range delayed fluorescence (DF) of pea chloroplasts, due to inhibition of the oxidizing side of photosystem II (PS II). The inhibition of the reducing side of PS II resulting in the suppression of millisecond DF takes place at much lower (∼0.01 μM) DCMU concentrations. The variation in the herbicide-affinities of the reducing and oxidizing sides of PS II is not the same for DCMU and phenol-type herbicides. The DCMU-affinity of the oxidizing side considerably increases and approximates that of the reducing side upon mild treatment of chloroplasts with oleic acid. Probably this is a result of some changes in the environment of the binding site at the oxidizing side. At DCMU concentrations higher than 1 mM, the chaotropic action of DCMU leads to the generation of millisecond luminescence which is not related to the functioning of the reaction centres.