Deregulation of electron flow within photosystem II in the absence of the PsbJ protein

The photosystem II (PSII) complex of photosynthetic oxygen evolving membranes comprises a number of small proteins whose functions remain unknown. Here we report that the low molecular weight protein encoded by the psbJ gene is an intrinsic component of the PSII complex. Fluorescence kinetics, oxygen flash yield, and thermoluminescence measurements indicate that inactivation of the psbJ gene in Synechocystis 6803 cells and tobacco chloroplasts lowers PSII-mediated oxygen evolution activity and increases the lifetime of the reduced primary acceptor Q(A)(-) (more than a 100-fold in the tobacco DeltapsbJ mutant). The decay of the oxidized S(2,3) states of the oxygen-evolving complex is considerably accelerated, and the oscillations of the Q(B)(-)/S(2,3) recombination with the number of exciting flashes are damped. Thus, PSII can be assembled in the absence of PsbJ. However, the forward electron flow from Q(A)(-) to plastoquinone and back electron flow to the oxidized Mn cluster of the donor side are deregulated in the absence of PsbJ, thereby affecting the efficiency of PSII electron flow following the charge separation process.     

Deletion of PsbM in tobacco alters the QB site properties and the electron flow within photosystem II

Photosystem II, the oxygen-evolving complex of photosynthetic organisms, includes an intriguingly large number of low molecular weight polypeptides, including PsbM. Here we describe the first knock-out of psbM using a transplastomic, reverse genetics approach in a higher plant. Homoplastomic Delta psbM plants exhibit photoautotrophic growth. Biochemical, biophysical, and immunological analyses demonstrate that PsbM is not required for biogenesis of higher order photosystem II complexes. However, photosystem II is highly light-sensitive, and its activity is significantly decreased in Delta psbM, whereas kinetics of plastid protein synthesis, reassembly of photosystem II, and recovery of its activity are comparable with the wild type. Unlike wild type, phosphorylation of the reaction center proteins D1 and D2 is severely reduced, whereas the redox-controlled phosphorylation of photosystem II light-harvesting complex is reversely regulated in Delta psbM plants because of accumulation of reduced plastoquinone in the dark and a limited photosystem II-mediated electron transport in the light. Charge recombination in Delta psbM measured by thermoluminescence oscillations significantly differs from the 2/6 patterns in the wild type. A simulation program of thermoluminescence oscillations indicates a higher Q(B)/Q(-)(B) ratio in dark-adapted mutant thylakoids relative to the wild type. The interaction of the Q(A)/Q(B) sites estimated by shifts in the maximal thermoluminescence emission temperature of the Q band, induced by binding of different herbicides to the Q(B) site, is changed indicating alteration of the activation energy for back electron flow. We conclude that PsbM is primarily involved in the interaction of the redox components important for the electron flow within, outward, and backward to photosystem II.     

Biochemical and structural analyses of a higher plant photosystem II supercomplex of a photosystem I-less mutant of barley. Consequences of a chronic over-reduction of the plastoquinone pool

Photosystem II of higher plants is a multisubunit transmembrane complex composed of a core moiety and an extensive peripheral antenna system. The number of antenna polypeptides per core complex is modulated following environmental conditions in order to optimize photosynthetic performance. In this study, we used a barley (Hordeum vulgare) mutant, viridis zb63, which lacks photosystem I, to mimic extreme and chronic overexcitation of photosystem II. The mutation was shown to reduce the photosystem II antenna to a minimal size of about 100 chlorophylls per photosystem II reaction centre, which was not further reducible. The minimal photosystem II unit was analysed by biochemical methods and by electron microscopy, and found to consist of a dimeric photosystem II reaction centre core surrounded by monomeric Lhcb4 (chlorophyll protein 29), Lhcb5 (chlorophyll protein 26) and trimeric light-harvesting complex II antenna proteins. This minimal photosystem II unit forms arrays in vivo, possibly to increase the efficiency of energy distribution and provide photoprotection. In wild-type plants, an additional antenna protein, chlorophyll protein 24 (Lhcb6), which is not expressed in viridis zb63, is proposed to associate to this minimal unit and stabilize larger antenna systems when needed. The analysis of the mutant also revealed the presence of two distinct signalling pathways activated by excess light absorbed by photosystem II: one, dependent on the redox state of the electron transport chain, is involved in the regulation of antenna size, and the second, more directly linked to the level of photoinhibitory stress perceived by the cell, participates in regulating carotenoid biosynthesis.     

Contrasting behavior of higher plant photosystem I and II antenna systems during acclimation

In this work we analyzed the photosynthetic apparatus in Arabidopsis thaliana plants acclimated to different light intensity and temperature conditions. Plants showed the ability to acclimate into different environments and avoid photoinhibition. When grown in high light, plants had a faster activation rate for energy dissipation (qE). This ability was correlated to higher accumulation levels of a specific photosystem II subunit, PsbS. The photosystem II antenna size was also regulated according to light exposure; smaller antenna size was observed in high light-acclimated plants with respect to low light plants. Different antenna polypeptides did not behave similarly, and Lhcb1, Lchb2, and Lhcb6 (CP24) are shown to undergo major levels of regulation, whereas Lhcb4 and Lhcb5 (CP29 and CP26) maintained their stoichiometry with respect to the reaction center in all growth conditions. The effect of acclimation on photosystem I antenna was different; in fact, the stoichiometry of any Lhca antenna proteins with respect to photosystem I core complex was not affected by growth conditions. Despite this stability in antenna stoichiometry, photosystem I light harvesting function was shown to be regulated through different mechanisms like the control of photosystem I to photosystem II ratio and the association or dissociation of Lhcb polypeptides to photosystem I.     

Structural determination of the photosystem II core complex from spinach

A photosystem II core complex was purified with high yield from spinach by solubilization with beta-dodecylmaltoside. The complex consisted of polypeptides with molecular mass 47, 43, 34, 31, 9 and 4 kDa and some minor components, as detected by silver-staining of polyacrylamide gels. There was no indication for the chlorophyll-a/b-binding, light-harvesting complex polypeptides. The core complex revealed electron-transfer activity (1,5-diphenylcarbazide----2,6-dichloroindophenol) of about 30 mumol reduced 2,6-dichloroindophenol/mg chlorophyll/h. The structural integrity was analyzed by electron microscopy. The detergent-solubilized protein complex has the shape of a triangular disk with a maximum diameter of 13 nm and a maximum height of 6.8 nm. The shape of this core complex differs considerably from that of cyanobacterial photosystem II membrane fragments, which are elongated particles. The structural differences between both the complexes of higher plants and cyanobacteria are discussed with special emphasis on their association with the antenna apparatus in the photosynthetic membranes.     

Inhibition of the oxygen-evolving complex of photosystem II and depletion of extrinsic polypeptides by nickel

The toxic effect of Ni²⁺ on photosynthetic electron transport was studied in a photosystem II submembrane fraction. It was shown that Ni²⁺ strongly inhibits oxygen evolution in the millimolar range of concentration. The inhibition was insensitive to NaCl but significantly decreased in the presence of CaCl₂. Maximal chlorophyll fluorescence, together with variable fluorescence, maximal quantum yield of photosystem II, and flash-induced fluorescence decays were all significantly declined by Ni²⁺. Further, the extrinsic polypeptides of 16 and 24 kDa associated with the oxygen-evolving complex of photosystem II were depleted following Ni²⁺ treatment. It was deduced that interaction of Ni²⁺ with these polypeptides caused a conformational change that induced their release together with Ca²⁺ from the oxygen-evolving complex of photosystem II with consequent inhibition of the electron transport activity.     

Conformational changes in photosystem II supercomplexes upon removal of extrinsic subunits

Photosystem II is a multisubunit pigment-protein complex embedded in the thylakoid membranes of chloroplasts. It consists of a large number of intrinsic membrane proteins involved in light-harvesting and electron-transfer processes and of a number of extrinsic proteins required to stabilize photosynthetic oxygen evolution. We studied the structure of dimeric supercomplexes of photosystem II and its associated light-harvesting antenna by electron microscopy and single-particle image analysis. Comparison of averaged projections from native complexes and complexes without extrinsic polypeptides indicates that the removal of 17 and 23 kDa extrinsic subunits induces a shift of about 1.2 nm in the position of the monomeric peripheral antenna protein CP29 toward the central part of the supercomplex. Removal of the 33 kDa extrinsic protein induces an inward shift of the strongly bound trimeric light-harvesting complex II (S-LHCII) of about 0.9 nm, and in addition destabilizes the monomer-monomer interactions in the central core dimer, leading to structural rearrangements of the core monomers. It is concluded that the extrinsic subunits keep the S-LHCII and CP29 subunits in proper positions at some distance from the central part of the photosystem II core dimer to ensure a directed transfer of excitation energy through the monomeric peripheral antenna proteins CP26 and CP29 and/or to maintain sequestered domains of inorganic cofactors required for oxygen evolution.     

Proteomic analysis of a highly active photosystem II preparation from the cyanobacterium Synechocystis sp. PCC 6803 reveals the presence of novel polypeptides

A highly active oxygen-evolving photosystem II (PSII) complex was purified from the HT-3 strain of the widely used cyanobacterium Synechocystis sp. PCC 6803, in which the CP47 polypeptide has been genetically engineered to contain a polyhistidine tag at its carboxyl terminus [Bricker, T. M., Morvant, J., Masri, N., Sutton, H. M., and Frankel, L. K. (1998) Biochim. Biophys. Acta 1409, 50-57]. These purified PSII centers had four manganese atoms, one calcium atom, and two cytochrome b(559) hemes each. Optical absorption and fluorescence emission spectroscopy as well as western immunoblot analysis demonstrated that the purified PSII preparation was devoid of any contamination with photosystem I and phycobiliproteins. A comprehensive proteomic analysis using a system designed to enhance resolution of low-molecular-weight polypeptides, followed by MALDI mass spectrometry and N-terminal amino acid sequencing, identified 31 distinct polypeptides in this PSII preparation. We propose a new nomenclature for the polypeptide components of PSII identified after PsbZ, which proceeds sequentially from Psb27. During this study, the polypeptides PsbJ, PsbM, PsbX, PsbY, PsbZ, Psb27, and Psb28 proteins were detected for the first time in a purified PSII complex from Synechocystis 6803. Five novel polypeptides were also identified in this preparation. They included the Sll1638 protein, which shares significant sequence similarity to PsbQ, a peripheral protein of PSII that was previously thought to be present only in chloroplasts. This work describes newly identified proteins in a highly purified cyanobacterial PSII preparation that is being widely used to investigate the structure, function, and biogenesis of this photosystem.     

Characterization of photosystem II mutants of Chlamydomonas reinhardii lacking the psbA gene

Summary We have examined 78 chloroplast mutants of Chlamydomonas reinhardii lacking photosystem II activity. Most of them are unable to synthesize the 32 Kdalton protein. Analysis of 22 of these mutants reveals that they have deleted both copies of the psbA gene (which codes for the 32 Kdalton protein) in their chloroplast genome. Although these mutants are able to synthesize and to integrate the other photosystem II polypeptides in the thylakoid membranes, they are unable to assemble a stable functional photosystem II complex. The 32 Kprotein appears therefore to play an important role not only in photosystem II function, but also in stabilizing this complex.     

Carotenoid photooxidation in photosystem II

Carotenoids are known to function as light-harvesting pigments and they play important roles in photoprotection in both plant and bacterial photosynthesis. These functions are also important for carotenoids in photosystem II. In addition, beta-carotene recently has been found to function as a redox intermediate in an alternate pathway of electron transfer within photosystem II. This redox role of a carotenoid in photosystem II is unique among photosynthetic reaction centers and stems from the very highly oxidizing intermediates that form in the process of water oxidation. In this minireview, an overview of the electron-transfer reactions in photosystem II is presented, with an emphasis on those involving carotenoids. The carotenoid composition of photosystem II and the physical methods used to study the structure of the redox-active carotenoid are reviewed. Possible roles of carotenoid cations in photoprotection of photosystem II are discussed.     

Characterization of a spinach photosystem II core preparation isolated by a simplified method

A photosystem II core complex from spinach exhibiting high rates of electron transport was obtained rapidly and in high yield by treatment of a Tris-extracted, O2-evolving photosystem II preparation with the detergent dodecyl-beta-D-maltoside. The core complex was essentially free of light-harvesting chlorophyll-protein and photosystem I polypeptides, and was highly enriched in the polypeptides associated with the photosystem II reaction center (45 and 49 kDa), cytochrome b559, and three polypeptides in the region 32-34 kDa. The photosystem II core complex contained two chlorophyll-proteins which had a slightly higher apparent molecular mass than CPa-1 and CPa-2. Additionally, a high-molecular-mass chlorophyll-protein complex termed CPa* was observed, which exhibited a low fluorescence yield when illuminated with ultraviolet light. This observation suggests that CPa* contains a functionally efficient quencher of chlorophyll fluorescence, possibly P680.     

Comparison of photosystem II complexes isolated from tobacco and two chlorophyll deficient tobacco mutants

A comparative study of photosystem II complexes isolated from tobacco (Nicotiana tabacum L. cv. John William's Broadleaf) which contains normal stacked thylakoid membranes, and from two chlorophyll deficient tobacco mutants (Su/su and Su/su var. Aurea) which have low stacked grana or essentially unstacked thylakoids with occasional membrane doublings, has been carried out. The corresponding photosystem II complexes had an O₂ evolving activity ranging from 290 (for the wild type) to 1100 mol O₂ x mg chlorophyll^-1 x h^-1 (for the mutant Su/su var. Aurea). The reduced photosynthetic unit size was also obvious in the mangenese and cytochrome_b559 content. The photosystem II complex from the wild type contained 4 Mn and 1 cytochrome_b559 per 200 to 280 chlorophylls, while the corresponding value for the mutant Su/su var. Aurea was 4 Mn and 1 cytochrome_b559 per 35 to 60 chlorophylls. We have also examined the polypeptide composition and show that the photosystem II complex from the wild type consisted of polypeptides of 48, 42, 33, 32, 30, 28, 23, 21, 18, 16 and 10 kDa, while the mutant complex mainly contained the polypeptides of 48, 42, 33, 32, 30, 28 and 10 kDa. In the mutant photosystem II complex the light-harvesting chlorophyll protein (peptide of 28 kDa) was reduced by a factor of 5 to 6 as compared to the wild type. With respect to the peptide composition and the photosynthetic unit size, the Triton-solubilized photosystem II complex from the mutant Su/su var. Aurea was very similar to O₂ evolving photosystem II reaction center core complexes.Abbreviations PS photosystem - chl chlorophyll - LHCP light-harvesting chlorophyll a/b protein complex     

Interaction of a phenolic inhibitor with photosystem II particles

Photosystem II particles have been prepared from spinach and Chlamydomonas reinhardii CW 15 thylakoids. Photosynthetic electron transport in these particles is inhibited by phenolic compounds like dinoseb, but not by atrazine and diuron. The labeling patterns obtained by photoaffinity labels derived from either atrazine (azido-atrazine) or the phenolic herbicide dinoseb (azido-dinoseb) were compared in photosystem II particles and thylakoids. Whereas azido-atrazine in thylakoids of spinach as well as of Chlamydomonas labels a 32-kilodalton peptide, this label does not react in photosystem II particle preparations. Azido-dinoseb, however, labels both the thylakoid membranes and the particles, predominantly polypeptides in the 40-53 kilodalton molecular weight region. Since the latter polypeptides are probably part of the reaction center of photosystem II, it is suggested that phenolic compounds have their inhibition site within the reaction center complex. This indicates that the atrazine-binding 32-kilodalton peptide is either absent or functionally inactive in photosystem II particles, whereas the phenol inhibitor-binding peptides are not.     

Cytochrome b-559 may function to protect photosystem II from photoinhibition

Although cytochrome b-559 is an integral component of the photosystem II complex (PSII), its function is unknown. Because cytochrome b-559 has been shown to be both photooxidized and photoreduced in PSII, one of several proposals is that it mediates cyclic electron transfer around PSII, possibly as a protective mechanism. We have used electron paramagnetic resonance spectroscopy to investigate the pathway of photooxidation of cytochrome b-559 in PSII and have shown that it proceeds via photooxidation of chlorophyll. We propose that this photooxidation of chlorophyll is the first step in the photoinhibition of PSII. The unique susceptibility of PSII to photoinhibition is probably due to the fact that it is the only reaction center in photosynthesis which generates an oxidant with a reduction potential high enough to oxidize chlorophyll. We propose that the function of cytochrome b-559 is to mediate cyclic electron transfer to rereduce photooxidized chlorophyll and protect PSII from photoinhibition. We also suggest that the chlorophyll(s) which are susceptible to photooxidation are analogous to the monomer chlorophylls found in the bacterial photosynthetic reaction center complex.     

The thermodynamics and kinetics of electron transfer between cytochrome b6f and photosystem I in the chlorophyll d-dominated cyanobacterium, Acaryochloris marina

We have investigated the photosynthetic properties of Acaryochloris marina, a cyanobacterium distinguished by having a high level of chlorophyll d, which has its absorption bands shifted to the red when compared with chlorophyll a. Despite this unusual pigment content, the overall rate and thermodynamics of the photosynthetic electron flow are similar to those of chlorophyll a-containing species. The midpoint potential of both cytochrome f and the primary electron donor of photosystem I (P(740)) were found to be unchanged with respect to those prevailing in organisms having chlorophyll a, being 345 and 425 mV, respectively. Thus, contrary to previous reports (Hu, Q., Miyashita, H., Iwasaki, I. I., Kurano, N., Miyachi, S., Iwaki, M., and Itoh, S. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 13319-13323), the midpoint potential of the electron donor P(740) has not been tuned to compensate for the decrease in excitonic energy in A. marina and to maintain the reducing power of photosystem I. We argue that this is a weaker constraint on the engineering of the oxygenic photosynthetic electron transfer chain than preserving the driving force for plastoquinol oxidation by P(740), via the cytochrome b(6)f complex. We further show that there is no restriction in the diffusion of the soluble electron carrier between cytochrome b(6)f and photosystem I in A. marina, at variance with plants. This difference probably reflects the simplified ultrastructure of the thylakoids of this organism, where no segregation into grana and stroma lamellae is observed. Nevertheless, chlorophyll fluorescence measurements suggest that there is energy transfer between adjacent photosystem II complexes but not from photosystem II to photosystem I, indicating spatial separation between the two photosystems.     

EPR characterization of an oxygen-evolving photosystem II preparation from the transformable cyanobacterium Synechocystis 6803.

The transformable cyanobacterium Synechocystis 6803 has a photosynthetic apparatus that is similar to that of plants. Because of the ease with which this organism can be genetically manipulated and isotopically labeled, Synechocystis has been used extensively in recent studies of electron transfer in the water-splitting complex, photosystem II. Here, we present the first EPR characterization of a highly active oxygen-evolving preparation from this organism. This preparation shows oxygen-evolution activities in the range from 2400-2600 mumol of O2/(mg of chlorophyll.h). We show that this preparation is stable enough for room temperature EPR studies. We then use this assay to show that the lineshapes of the D+ and Z+ tyrosine radicals are identical in this preparation, as has been observed in photosystem II complexes from a wide variety of photosynthetic species. We also present the first multiline EPR spectrum that has been observed from the Synechocystis manganese cluster.     

Electron transport regulates exchange of two forms of photosystem II D1 protein in the cyanobacterium Synechococcus.

Synechococcus sp. PCC 7942 modulates photosynthetic function by transiently replacing the constitutive D1 photosystem II protein, D1:1, with an alternate form, D1:2, to help counteract photoinhibition under excess light. We show that a temperature drop from 37 to 25 degrees C also drives D1:1/D1:2 exchange under constant, moderate light. Chilling or light-induced D1 exchange results from rapid loss of psbAI message coding for D1:1 and accumulation of psbAII and psbAIII messages coding for D1:2. During chilling, a large pool of a novel form, D1:2*, transiently accumulates, distinguishable from normal D1 by an increase in apparent molecular mass. D1:2* is not phosphorylated and is probably a functionally inactive, incompletely processed precursor. After acclimation to 25 degrees C, D1:2* disappears and D1:1 again predominates, although substantial D1:2 remains. Partial inhibition of electron transport under constant, moderate light also triggers the D1 exchange process. These treatments all increase excitation pressure on photosystem II relative to electron transport. Therefore, information from photosynthetic electron transport regulates D1 exchange without any requirement for a change in light intensity or quality, possibly via a redox sensing mechanism proximal to photosystem II.     

Polypeptides of photosystem II and their role in oxygen evolution

The linear, four-step oxidation of water to molecular oxygen by photosystem II requires cooperation between redox reactions driven by light and a set of redox reactions involving the S-states within the oxygen-evolving complex. The oxygenevolving complex is a highly ordered structure in which a number of polypeptides interact with one another to provide the appropriate environment for productive binding of cofactors such as manganese, chloride and calcium, as well as for productive electron transfer within the photoact. A number of recent advances in the knowledge of the polypeptide structure of photosystem II has revealed a correlation between primary photochemical events and a core complex of five hydrophobic polypeptides which provide binding sites for chlorophyll a, pheophytin a, the reaction center chlorophyll (P680), and its immediate donor, denoted Z. Although the core complex of photosystem II is photochemically active, it does not possess the capacity to evolve oxygen. A second set of polypeptides, which are water-soluble, have been discovered to be associated with photosystem II; these polypeptides are now proposed to be the structural elements of a special domain which promotes the activities of the loosely-bound cofactors (manganese, chloride, calcium) that participate in oxygen evolution activity. Two of these proteins (whose molecular weights are 23 and 17 kDa) can be released from photosystem II without concurrent loss of functional manganese; studies on these proteins and on the membranes from which they have been removed indicate that the 23 and 17 kDa species from part of the structure which promotes retention of chloride and calcium within the oxygen-evolving complex. A third water-soluble polypeptide of molecular weight 33 kDa is held to the photosystem II core complex by a series of forces which in some circumstances may include ligation to manganese. The 33 kDa protein has been studied in some detail and appears to promote the formation of the environment which is required for optimal participation by manganese in the oxygen evolving reaction. This minireview describes the polypeptides of photosystem II, places an emphasis on the current state of knowledge concerning these species, and discusses current areas of uncertainty concerning these important polypeptides.Abbreviations A 23187 ionophore that exchanges divalent cations with H⁺ - Chl chlorophyll - cyt cytochrome - DCPIP dichlorophenolindophenol - DPC diphenylcarbazide - EGTA ethyleneglycoltetraacetic acid - P680 the chlorophyll a reaction center of photosystem II - pheo pheophytin - PQ plastoquinone - PS photosystem - Q_A and Q_B primary and secondary plastoquinone electron acceptors of photosystem II - Sn (n=0, 1, 2, 3, 4) charge accumulating state of the oxygen evolving system - Signals II_vf, II_f and II_s epr-detectable free radicals associated with the oxidizing side of photosystem II - Z primary electron donor to the photosystem II reaction center The survey of literature for this review ended in September, 1984.     

Different roles of alpha- and beta-branch xanthophylls in photosystem assembly and photoprotection

Xanthophylls (oxygenated carotenoids) are essential components of the plant photosynthetic apparatus, where they act in photosystem assembly, light harvesting, and photoprotection. Nevertheless, the specific function of individual xanthophyll species awaits complete elucidation. In this work, we analyze the photosynthetic phenotypes of two newly isolated Arabidopsis mutants in carotenoid biosynthesis containing exclusively alpha-branch (chy1chy2lut5) or beta-branch (chy1chy2lut2) xanthophylls. Both mutants show complete lack of qE, the rapidly reversible component of nonphotochemical quenching, and high levels of photoinhibition and lipid peroxidation under photooxidative stress. Both mutants are much more photosensitive than npq1lut2, which contains high levels of viola- and neoxanthin and a higher stoichiometry of light-harvesting proteins with respect to photosystem II core complexes, suggesting that the content in light-harvesting complexes plays an important role in photoprotection. In addition, chy1chy2lut5, which has lutein as the only xanthophyll, shows unprecedented photosensitivity even in low light conditions, reduced electron transport rate, enhanced photobleaching of isolated LHCII complexes, and a selective loss of CP26 with respect to chy1chy2lut2, highlighting a specific role of beta-branch xanthophylls in photoprotection and in qE mechanism. The stronger photosystem II photoinhibition of both mutants correlates with the higher rate of singlet oxygen production from thylakoids and isolated light-harvesting complexes, whereas carotenoid composition of photosystem II core complex was not influential. In depth analysis of the mutant phenotypes suggests that alpha-branch (lutein) and beta-branch (zeaxanthin, violaxanthin, and neoxanthin) xanthophylls have distinct and complementary roles in antenna protein assembly and in the mechanisms of photoprotection.     

Efficient assembly of photosystem II in Chlamydomonas reinhardtii requires Alb3.1p, a homolog of Arabidopsis ALBINO3

Alb3 homologs Oxa1 and YidC have been shown to be required for the integration of newly synthesized proteins into membranes. Here, we show that although Alb3.1p is not required for integration of the plastid-encoded photosystem II core subunit D1 into the thylakoid membrane of Chlamydomonas reinhardtii, the insertion of D1 into functional photosystem II complexes is retarded in the Alb3.1 deletion mutant ac29. Alb3.1p is associated with D1 upon its insertion into the membrane, indicating that Alb3.1p is essential for the efficient assembly of photosystem II. Furthermore, levels of nucleus-encoded light-harvesting proteins are vastly reduced in ac29; however, the remaining antenna systems are still connected to photosystem II reaction centers. Thus, Alb3.1p has a dual function and is required for the accumulation of both nucleus- and plastid-encoded protein subunits in photosynthetic complexes of C. reinhardtii.