Oxygen-Evolving System of Inactive Photosystem II Complexes in Chlorella

Incubation of green alga Chlorella pyrenoidosa Chick in darkness at 37–38°C for 10–30 h resulted in inactivation of the oxygen-evolving complex (OEC): the maximum yield of oxygen evolution during a series of short light flashes shifted from the third to the fifth flash; the transition of S₂- and S₃-states of OEC to a stable S₁-state was markedly accelerated. This inactivation of OEC was accompanied by the accumulation of inactive complexes of photosystem II (PSII), in which the reduction of primary quinone acceptor and the conversion into the closed state occurred with a low efficiency, even in the presence of 5 M DCMU. The treatment of light-grown algal cells with hydroxylamine impaired OEC functioning, in similarity to the effect of dark incubation, but caused no accumulation of inactive PSII complexes. We conclude that the inactivation of OEC is not the cause of the inactivation of PSII complex. The decline in the efficiency of electron-transport reactions, both on the donor and acceptor sides of PSII may be related to modification of major proteins in the PS II reaction center.     

Colocalization of Polyphenol Oxidase and Photosystem II Proteins

Polyphenol oxidase (PPO) appears to be ubiquitous in higher plants but, as yet, no function has been ascribed to it. Herein, we report on the localization of PPO based upon biochemical fractionation of chloroplast membranes in Vicia faba (broad bean) into various complexes and immunocytochemical electron microscopic investigations. Sucrose density gradient fractionations of thylakoid membranes after detergent solubilization reveals that PPO protein (by reactivity with anti-PPO antibody) and activity (based upon ability to oxidize di-dihydroxyphenylalanine) are found only in fractions enriched in photosystem II (PSII). Furthermore, of the PSII particles isolated using three different protocols utilizing several plant species, all had PPO. Immunogold localization of PPO on thin sections reveals exclusive thylakoid labeling with a distribution pattern consistent with other PSII proteins (80% grana, 20% stroma). These data strongly indicate that PPO is at least peripherally associated with the PSII complex.     

Energy transfer between Photosystem II and Photosystem I in chloroplasts

A model for the photochemical apparatus of photosynthesis is presented which accounts for the fluorescence properties of Photosystem II and Photosystem I as well as energy transfer between the two photosystems. The model was tested by measuring at ?196 °C fluorescence induction curves at 690 and 730 nm in the absence and presence of 5 mM MgCl₂ which presumably changes the distribution of excitation energy between the two photosystems. The equations describing the fluorescence properties involve terms for the distribution of absorbed quanta, α, being the fraction distributed to Photosystem I, and β, the fraction to Photosystem II, and a term for the rate constant for energy transfer from Photosystem II to Photosystem I,k_T(II→I). The data, analyzed within the context of the model, permit a direct comparison of α andk_T(II→I) in the absence (?) and presence (+) of Mg²⁺:α/^?α⁺= 1.2andk/^?_T(II→I)k⁺_T(II→I)= 1.9. If the criterion thatα + β = 1 is applied absolute values can be calculated: in the presence of Mg²⁺,a⁺ = 0.27 and the yield of energy transfer,φ⁺_T(II→I) varied from 0.065 when the Photosystem II reaction centers were all open to 0.23 when they were closed. In the absence of Mg²⁺,α^? = 0.32 andφ_T(II→I) varied from 0.12 to 0.28.The data were also analyzed assuming that two types of energy transfer could be distinguished; a transfer from the light-harvseting chlorophyll of Photosystem II to Photosystem I,k_T(II→I), and a transfer from the reaction centers of Photosystem II to Photosystem I,k_t(II→I). In that caseα/^?α⁺= 1.3,k/^?_T(II→I)k⁺_T(II→I)= 1.3 andk/^?_t(II→I)k⁺_(tII→I)= 3.0. It was concluded, however, that both of these types of energy transfer are different manifestations of a single energy transfer process.     

Distribution of Thylakoid Proteins between Stromal and Granal Lamellae in Spirodela : Dual Location of Photosystem II Components

We have quantified the lateral distribution of 12 thylakoid proteins of Spirodela oligorrhiza by immunoblot analysis of detergent-derived granal and stromal lamellae. The immunological, ultrastructural, cytochemical, and biophysical measurements each indicated the expected overall separation of photosystem II (PSII) and photosystem I (PSI) components; however, certain proteins were not completely localized to one lamellar fraction. The apoproteins of the light harvesting chlorophyll a/b complex, subunit 1 of PSI and the components of the PSII reaction center (the 32 kilodalton, D2, and cytochrome b₅₅₉ proteins) were dually located between granal and stromal lamellae. Proteins associated exclusively with one of the membrane types were: in granal lamellae, the 43 and 51 kilodalton PSII proteins, and in stromal lamellae, the α and β subunits of the proton ATPase.     

Control of Enzyme Activities in Cotton Cotyledons during Maturation and Germination: II. Glyoxysomal Enzyme Development in Embryos

The sequence of glyoxysomal enzyme development was investigated in cotyledons of cotton (Gossypium hirsutum L. cv. Deltapine 16) embryos from 16 to 70 days after anthesis (DAA). Catalase, malate dehydrogenase, and citrate condensing enzyme activities were barely detectable prior to 22 DAA, but showed dramatic increases from 22 to 50 DAA. Development of malate synthase activity, however, was delayed during this period, rising to peak activity from 45 to 50 DAA (just prior to desiccation) in the absence of any detectable isocitrate lyase activity. Substantial activities of all of these enzymes (except isocitrate lyase) persisted in the dry seeds. Isopycnic centrifugations on sucrose gradients demonstrated that the enzymes were compartmentalized within particles increasing in buoyant density with time of development (1.226 to 1.245 grams per cubic centimeter from 22 to 50 DAA). Of particular significance were the observations in 22-day embryos of smooth surfaced membrane dilations of rough endoplasmic reticulum having cytochemical catalase reactivity, and the demonstrations of catalase activities in microsomal fractions isolated throughout the 16- to 50-DAA period. Our data do not allow determination of the mechanism(s) for enzyme activation and/or addition to previously existing or newly formed microbodies, but do show that development and acquisition of enzyme activities within glyoxysomes occur sequentially and thus are not regulated in concert as previously thought.     

Interaction of Benzoquinones with QA- and QB- in Oxygen-Evolving Photosystem II Particles from the Thermophilic Cyanobacterium Synechococcus elongatus

The interactions of benzoquinones with the reduced forms ofthe bound plastoquinone acceptors, Q_A and Q_B, were studied withoxygen-evolving photosystem II (PS II) particles from the thermophiliccyanobacterium Synechococcus elongatus, which largely lack poolplastoquinone molecules [Takahashi and Katoh (1986) Biochim.Biophys. Acta 845: 183]. Oxygen evolution in the presence ofvarious electron acceptors was determined and flash-inducedchanges in absorbance in the blue region were analyzed in termsof difference spectra, dependence on the concentration of benzoquinoneand on temperature, and sensitivity to 3-(3,4-dichlorophenyl)-1,1-dimethylurea(DCMU). The more hydrophobic the quinone molecule, the higherwas the rate of oxygen evolution, and the maximum rate of 3,000µmoles O₂.(mg chlorophyll)^–1.h^–1 was recordedin the presence of phenyl- and dichloro-p-benzoquinones. DCMUinhibited oxygen evolution by more than 95%. However, spectrophotometricstudies revealed that, even though electrons were transferredto benzoquinones predominantly via the direct oxidation of by added benzoquinones occurred in such a way as to indicate thatabout 40% of PS II reaction centers were not associated withfunctional Q_B sites. was very stable in the presence of ferricyanide. However, benzoquinonesinduced the slow oxidation of . The characteristics of the benzoquinone reductioin in thePS II preparation is discussed. ¹Present address: Department of Life Sciences, Faculty of Science,Himeji Institute of Technology, Shosha 2167, Himejishi, Hyogo-ken,671-22 Japan (Received May 8, 1990; Accepted August 14, 1990)     

Changes in Paramagnetic Manganese (Mn2+) and Related Enzyme Activities in Isolated Cucumber Cotyledons During Growth: II. EFFECT OF 6-FURFURYLAMINOPURINE

Changes in the activities of IAA oxidase, peroxidase, ascorbicacid utilization (AAU), and in the level of paramagnetic manganese(Mn²⁺) have been studied during kinetin-induced growth of theisolated cucumber cotyledons in light or in dark. In kinetin-treatedcotyledons exposed to light, inhibition in the level of paramagneticmanganese corresponds with an enhancement in IAA oxidase activity.The level of paramagnetic manganese shows an inverse correlationwith IAA oxidase activity. In darkness the level of Mn²⁺ doesnot show the same correlation with IAA oxidase activity as inthe light. Kinetin stimulates peroxidase activity both in thelight and in darkness. Enhancement of IAA oxidase activity andno corresponding change in the level of paramagnetic manganeseindicates that the oxidation of IAA in dark-grown, kinetin-treatedcotyledons is brought about by peroxidase. It appears that thephenolic cofactors required for the oxidation of manganese andIAA may be limiting in kinetin-treated cotyledons in darkness.Thus in the light, IAA oxidation seems to be brought about byperoxidase as well as manganese, whereas in darkness it is mediatedby peroxidase alone. Increase in IAA oxidase activity duringkinetin-induced growth of the isolated cotyledons is incompatiblewith the idea that increased IAA oxidase activity would limitthe availability of auxin for growth. Kinetin does not mimicthe action of light on IAA oxidase activity; on the contrary,it removes the inhibitory effect of light on IAA oxidase activityprobably through the synthesis of IAA oxidase activators.     

Changes in Paramagnetic Manganese (Mn(II)) and Related Enzyme Activities in Isolated Cucumber Cotyledons During Growth: I. EFFECT OF GIBBERELLIC ACID

Changes in the activities of IAA oxidase, peroxidase, and ascorbicacid utilization (AAU) and in the level of paramagnetic manganese(Mn(II)) have been studied during GA₃-induced growth of theisolated cucumber cotyledons in light or dark. In dark-growncotyledons, where GA₃ fails to evoke any growth response, aninitial decline in the Mn(II) content during the first 8 h isfollowed by a rise which approaches the control value between12 and 24 h. A marked, almost linear increase is observed inMn(II) levels from 48 h onwards. The initial decline in Mn(II)is accompanied by an increase in IAA oxidase activity and decreasedAAU. The large increases obtained at later times, viz. 48 and72 h, appear to be related primarily to the release of manganesedue to the degradation of protein bodies which proceeds underthe action of GA and also to increased AAU. The IAA oxidaseand peroxidase activities are close to control during this period.In light, however, IAA oxidase and peroxidase activities showmarked increase during the first 4 h but no significant changesare observed in Mn(II) level or AAU. The peak of Mn(II) observedat 8 h corresponds with lower activities of IAA oxidase andperoxidase but an increase in AAU. The changes in Mn(II) closelycorrespond with the changes in AAU in light, peroxidase andIAA oxidase activities keeping close to the control. It appearsthat GA action is related to increased Mn(II) due to increasedAAU which may be coupled to the peroxidase-IAA oxidase system.The study confirms the postulate given by us in an earlier paper.     

A Comparative Study of Quantitative Relationship between Phycobiliproteins and Photosystem II in Cyanobacteria and Red Algae

Quantitative relationship between phycobiliprotein (PBP) andPS II contents was compared for 10 cyanobacterial and 3 redalgal strains. Following results were obtained: (1) contentof PBP per PS II was always equal to that of one phycobilisome(PBS) when PBS was hemidiscoidal type, (2) unusually high ratiosbetween antenna PBP [phycoerythrin (PE) and phycocyanin (PC)]and allophycocyanin contents were found in the PE-containingcyanobacteria Phormidium persicinum, Phormidium sp. NIBB 1052and Synechococcus sp. NIBB 1059 and the red alga Porphyra yezoensissuggesting that PBS in these organisms is not ordinary hemidiscoidaltype, and (3) the PBP content per PS II in Synechococcus NIBB1059 and P. yezoensis was as small as 14 to 13 of that of onePBS similarly to the case of Porphyridium cruentum. Resultssuggest that (1) hemidiscoidal PBS is the antenna for only onePS II, but (2) in some of organisms containing non-hemidiscoidalPBS, one PBS becomes a common antenna for plural PS II (around4), and (3) such PBS occurs not only in red algae but also incyanobacteria. ¹ Present address: Department of Biology, Faculty of Science,Toho University, Funabashi, Chiba 274, Japan. 1219 (Received April 15, 1987; Accepted July 11, 1987)     

Preparation and Characterization of Heat-Stable and Very Active Oxygen-Evolving Photosystem II Particles from the Thermophilic Cyanobacterium, Synechococcus elongatu

Very active and heat-stable oxygen-evolving photosystem II particleswere isolated from the thermophilic cyanobacterium Synechococcuselongatus by treatment of thylakoid membranes with a non-ionicdetergent, sucrose monolaurate (SML). The particles were analyzedin a comparison with photosystem II particles prepared withß-octylglucoside (OG). The two preparations had similarpolypeptide compositions, which were caracterized by high levelsof polypeptides from phycobilisomes. The ratio of chlorophylla to QA was 45 and there were four Mn atoms and one tightlybound Ca²⁺ ion per QA in the particles prepared with SML. Thepreparations were thermophilic, showing substantial rates ofoxygen evolution at temperatures up to 60°C. The maximumrates attained at 45°C were as high as 6.0 mmoles O₂ mg^–1Chl h^–1. PS II particles prepared with OG were similarlythermostable but were less active in oxygen evolution at alltemperatures examined. Kinetic analysis of flash-induced absorptiontransients revealed that about 22% and 28% of photosystem IIreaction centers were not associated with the functional QBsite in the SML- and OG-particles, respectively. When correctedfor the inactive reaction centers, the maximum rates of oxygenevolution by SML- and OG-particles were 7.7 and 7.0 mmoles O₂mg^–1 Chl h^–1, which correspond to half times of1.9 and 2.1 ms for the first-order electron transfer, respectively.Comparison of these half times with those of the S-state transitionand the release of oxygen indicates that the overall photosystemII electron transport is limited by the reduction of added electronacceptors and not by release of oxygen. ³On leave from National Chemical Laboratory for Industry, Higashi1-1, Tsukuba, Ibaraki 305     

The Origin of the Multiline and g = 4.1 Electron Paramagnetic Resonance Signals from the Oxygen-Evolving System of Photosystem II

Continuous illumination at 200 K of photosystem (PS) II-enriched membranes generates two electron paramagnetic resonance (EPR) signals that both are connected with the S₂ state: a multiline signal at g 2 and a single line at g = 4.1. From measurements at three different X-band frequencies and at 34 GHz, the g tensor of the multiline species was found to be isotropic with g = 1.982. It has an excited spin multiplet at ~30 cm^-1, inferred from the temperature-dependence of the linewidth. The intensity ratio of the g = 4.1 signal to the multiline signal was found to be almost constant from 5 to 23 K. Based on these findings and on spin quantitation of the two signals in samples with and without 4% ethanol, it is concluded that they arise from the ground doublets of paramagnetic species in different PS II centers. It is suggested that the two signals originate from separate PS II electron donors that are in a redox equilibrium with each other in the S₂ state and that the g = 4.1 signal arises from monomeric Mn(IV).     

Assembly of Newly Imported Oxygen-Evolving Complex Subunits in Isolated Chloroplasts: Sites of Assembly and Mechanism of Binding

We have examined the assembly of the nuclear-encoded subunits of the oxygen-evolving complex (OEC) after their import into isolated intact chloroplasts. We showed that all three subunits examined (OE33, OE23, and OE17) partition between the thylakoid lumen and a site on the inner surface of the thylakoid membrane after import in a homologous system (e.g., pea or spinach subunits into pea or spinach chloroplasts, respectively). Although some interspecies protein import experiments resulted in OEC subunit binding, maize OE17 did not bind thylakoid membranes in chloroplasts isolated from peas. Newly imported OE33 and OE23 were washed from the membranes at the same concentrations of urea and NaCl as the native, indigenous proteins; this observation suggests that the former subunits are bound productively within the OEC. Inhibition of neither chloroplast protein synthesis nor light- or ATP-dependent energization of the thylakoid membrane significantly affected these assembly reactions, and we present evidence suggesting that incoming subunits actively displace those already bound to the thylakoid membrane. Transport of OE33 took place primarily in the stromal-exposed membranes and proceeded through a protease-sensitive, mature intermediate. Initial binding of OE33 to the thylakoid membrane occurred primarily in the stromal-exposed membranes, from where it migrated with measurable kinetics to the granal region. In contrast, OE23 assembly occurred in the granal membrane regions. This information is incorporated into a model of the stepwise assembly of oxygen-evolving photosystem II.     

Binding Affinities of Oxidized and Reduced Forms of Tetrahalogenated Benzoquinones to the QB Site in Oxygen-Evolving Photosystem II Particles from Synechococcus elongatus

Binding affinities of the Q_B site for four tetrahalogenatedbenzoquinones (THBQs) were investigated by measuring their abilityto serve as electron acceptors or act as inhibitors of oxygenevolution in Synechococcus photosystem II particles. Iodanil,bromanil and chloranil but not fluoranil induced a rapid oxidationof Q_A^– and doubled the area over the fluorescence inductioncurve, indicating dark oxidation of Q₄₀₀. Analyses of thesetwo THBQ-induced reactions and inhibition of the acceleratedQ_A^– oxidation by DCMU yielded binding constants of thequinones comparable to those determined from measurements ofoxygen evolution. Generally, THBQs bound tightly to the Q_B site.However, the binding affinity varied in a wide range with THBQs.The Q_B site bound iodanil with an extremely high affinity butfluoranil relatively weakly. The hydroquinone forms of the THBQsalso bound to the Q_B site and inhibited Q_A^– oxidationby Q_B. The concentrations of the hydroquinones required for50% inhibition of Q_A^– oxidation suggest that the Q_B sitebinds the hydroquinones more weakly than the corresponding quinonesexcept for fluoranil, which binds to the Q_B site more tightlyin its reduced form than in oxidized one. The abilities of THBQsto function as electron acceptors or inhibitors of oxygen evolution,and as oxidants of Q₄₀₀ in the dark, are discussed in relationto the binding affinities of the quinones to the Q_B site. ⁴Present address: Department of Biology, Faculty of Science,Toho University. Miyama 2-2-1, Funabashi, 274 Japan     

Relationship between O2 evolution capacity and cytochrome b-559 high-potential form in Photosystem II particles

As previously shown for inside-out vesicles by Larsson et al. (Larsson, C., Jansson, C., Ljungberg, U.L., Åkerlund, H.E. and Anderson, B. (1984) in Advances in Photosynthesis Research, Vol. I, pp. 363–366 (Sybesma C., ed.), Martinus Nijhoff/Dr. W. Junk Publishers, Dordrecht, The Netherlands), we observed that NaCl 1 M washing of Photosystem II particles prepared by Triton X-100 treatment of spinach thylakoids induces both an inactivation of oxygen evolution and transformation of cytochrome b-559 from its high-potential to its low-potential form. A partial reactivation of water oxidation by 24 kDa polypeptide refixation is accompanied by a partial restoration of the cytochrome b-559 high-potential (HP) form. In contrast, reconstitution of water splitting by Ca²⁺ addition is not associated to a reestablishment of the cytochrome (HP) form. We conclude that cytochrome b-559 HP plays no role in water oxidation.     

钙通道蛋白与植物抗盐性和抗冷性关系研究进展

植物钙通道蛋白几乎在植物生长发育的所有阶段都是必需的,它们参与细胞内钙离子浓度的调控,在植物细胞内钙离子的跨膜转运过程中起着极其重要的作用;它们同时调控植物细胞和组织的极性生长,参与植物应对一系列不同逆境胁迫因素的适应性反应,在植物抗逆方面同样起着极其重要的作用.本文对近年来国内外有关不同钙通道蛋白的性质及其在植物抗冷性和抗盐性中的作用研究进展进行综述,为在生理水平和分子水平上深入阐明植物钙通道蛋白参与植物抗逆性的机理提供信息资料.     

Cooperation of LPA3 and LPA2 Is Essential for Photosystem II Assembly in Arabidopsis

Photosystem II (PSII) is a multisubunit membrane protein complex that is assembled in a sequence of steps. However, the molecular mechanisms responsible for the assembly of the individual subunits into functional PSII complexes are still largely unknown. Here, we report the identification of a chloroplast protein, Low PSII Accumulation3 (LPA3), which is required for the assembly of the CP43 subunit in PSII complexes in Arabidopsis (Arabidopsis thaliana). LPA3 interacts with LPA2, a previously identified PSII CP43 assembly factor, and a double mutation of LPA2 and LPA3 is more deleterious for assembly than either single mutation, resulting in a seedling-lethal phenotype. Our results indicate that LPA3 and LPA2 have overlapping functions in assisting CP43 assembly and that cooperation between LPA2 and LPA3 is essential for PSII assembly. In addition, we provide evidence that LPA2 and LPA3 interact with Albino3 (Alb3), which is essential for thylakoid protein biogenesis. Thus, the function of Alb3 in some PSII assembly processes is probably mediated through interactions with LPA2 and LPA3.Oxygenic photosynthesis, in which oxygen and organic carbon are produced from water and carbon dioxide using sunlight, provides energy for nearly all living organisms on Earth. Four major multiprotein complexes, located in thylakoid membranes, are responsible for the capture of light and its conversion to chemical energy in eukaryotic photosynthetic organisms: PSI, PSII, cytochrome b₆/f, and ATP synthase (Wollman et al., 1999; Nelson and Yocum, 2006). PSII catalyzes one of the most important of all biochemical reactions, the light-induced transfer of electrons from water to plastoquinone, which generates most of the oxygen in the Earth’s atmosphere. PSII consists of more than 20 subunits in higher plants (Wollman et al., 1999; Iwata and Barber, 2004; Nelson and Yocum, 2006). The PSII reaction center consists of the D1 and D2 proteins, the α- and β-subunits of cytochrome b₅₅₉, and the PsbI protein, and the D1 and D2 heterodimers bind all the redox components essential for the primary charge separation (Nanba and Satoh, 1987). The PSII core complex additionally contains CP47, CP43, the oxygen-evolving complex, and several low molecular mass proteins (Wollman et al., 1999; Nelson and Yocum, 2006). CP47 and CP43, two inner chlorophyll a-binding proteins, are closely associated with, and located on opposite sides of, the PSII reaction center (Hankamer et al., 1999). The functional form of PSII cores in thylakoid membranes is dimeric and is associated with light-harvesting complex (LHC). In PSII-LHCII supercomplexes, PSII core dimers are surrounded by LHCII trimers, which consist of Lhcb1 and Lhcb2 proteins (Wollman et al., 1999; Iwata and Barber, 2004; Nelson and Yocum, 2006).Our knowledge of the molecular mechanisms involved in the biogenesis and assembly of PSII in the thylakoid membranes is still limited, although the structure and function of PSII have been extensively studied. Genetic and biochemical studies have elucidated several distinct steps that occur in PSII assembly. D2 and cytochrome b₅₅₉ form an initial complex, which serves as a receptor for the cotranslational assembly of D1 (Adir et al., 1990; van Wijk et al., 1997; Müller and Eichacker, 1999; Zhang et al., 1999). The next step involves the association of CP47 with the PSII reaction center (Zhang et al., 1999; Rokka et al., 2005), while CP43 is synthesized independently and then continuously associates and dissociates with PSII (de Vitry et al., 1989; Zhang et al., 2000). The biogenesis of PSII involves “a control by epistasy of synthesis” process (Minai et al., 2006). D2 is required for D1 synthesis, which itself is needed for CP47 synthesis. However, many aspects of the processes involved in the oligomerization and coordination of the various PSII subunits are still unclear (Rochaix, 2001). Due to the structural complexity of PSII, its assembly consists of multiple assembly steps, which is likely to require the participation of a number of assembly factors.Several assembly factors involved in the biosynthesis and assembly of the PSII complex have been identified recently. For instance, the thylakoid lumen protein HCF136 is known to be required for the formation of PSII, since the hcf136 mutant is capable of synthesizing plastid-encoded proteins, but it does not appear to accumulate any stable PSII complexes, due to blockage of the assembly of the PSII reaction center (Meurer et al., 1998; Plücken et al., 2002). Alb3.1, a homolog of Arabidopsis (Arabidopsis thaliana) Albino3 (Alb3), is essential for the efficient assembly of PSII in Chlamydomonas reinhardtii, probably through interactions with D1 following its insertion (Ossenbühl et al., 2004), and another Alb3 homolog, Alb3.2, appears to be required for photosystem assembly in Chlamydomonas (Göhre et al., 2006). Coimmunoprecipitation analysis has shown that Alb3.1 and Alb3.2 interact directly, while Alb3.2 reportedly interacts with the PSI and PSII reaction centers proteins (Göhre et al., 2006). The lumenal immunophilins, AtCYP38 and FKBP20-2, have also been shown to be involved in PSII assembly (Lima et al., 2006; Fu et al., 2007; Sirpiö et al., 2008). In addition, we recently identified two PSII assembly factors, Low PSII Accumulation1 (LPA1) and LPA2, involved in PSII assembly. The LPA1 protein appears to be an integral membrane chaperone required for efficient assembly of the PSII core complex, probably through direct interaction with D1 (Peng et al., 2006). LPA2, which interacts with Alb3, forms a protein complex that assists CP43 assembly within PSII (Ma et al., 2007). These findings suggest that each stage of the PSII assembly process is assisted by one or more specific assembly factors, most of which have not yet been identified.Here, we report the identification of a lpa3 mutant with reduced levels of PSII. Functional characterization points to the possible role of LPA3 in assisting CP43 assembly within PSII. In addition, biochemical and genetic analyses indicate that an assembly complex of LPA3 and LPA2 is essential for PSII assembly.