Truncation of the COOH-terminal domain of the psbE gene product in Synechocystis sp. PCC 6803: requirements for photosystem II assembly and function

Structural, functional and auxiliary proteins of photosystem II

Photosystem II (PSII) is the water-splitting enzyme complex of photosynthesis and consists of a large number of protein subunits. Most of these proteins have been structurally and functionally characterized, although there are differences between PSII of plants, algae and cyanobacteria. Here we catalogue all known PSII proteins giving a brief description, where possible of their genetic origin, physical properties, structural relationships and functions. We have also included details of auxiliary proteins known at present to be involved in the in vivo assembly, maintenance and turnover of PSII and which transiently bind to the reaction centre core complex. Finally, we briefly give details of the proteins which form the outer light-harvesting systems of PSII in different types of organisms.

The low molecular mass subunits of the photosynthetic supracomplex, photosystem II

The photosystem II (PSII) complex is located in the thylakoid membrane of higher plants, algae and cyanobacteria and drives the water oxidation process of photosynthesis, which splits water into reducing equivalents and molecular oxygen by solar energy. Electron and X-ray crystallography analyses have revealed that the PSII core complex contains between 34 and 36 transmembrane alpha-helices, depending on the organism. Of these helices at least 12-14 are attributed to low molecular mass proteins. However, to date, at least 18 low molecular mass (<10 kDa) subunits are putatively associated with the PSII complex. Most of them contain a single transmembrane span and their protein sequences are conserved among photosynthetic organisms. In addition, these proteins do not have any similarity to any known functional proteins in any type of organism, and only two of them bind a cofactor. These findings raise intriguing questions about why there are so many small protein subunits with single-transmembrane spans in the PSII complex, and their possible functions. This article reviews our current knowledge of this group of proteins. Deletion mutations of the low molecular mass subunits from both prokaryotic and eukaryotic model systems are compared in an attempt to understand the function of these proteins. From these comparisons it seems that the majority of them are involved in stabilization, assembly or dimerization of the PSII complex. The small proteins may facilitate fast dynamic conformational changes that the PSII complex needs to perform an optimal photosynthetic activity.

Photosynthetic water oxidation in cytochrome b(559) mutants containing a disrupted heme-binding pocket

The role of cytochrome b(559) in photosynthetic oxygen evolution has been investigated in three chloroplast mutants of Chlamydomonas reinhardtii, in which one of the two histidine axial ligands to the heme, provided by the alpha subunit, has been replaced by the residues methionine, tyrosine, and glutamine. Photosystem two complexes functional for oxygen evolution could be assembled in the methionine and tyrosine mutants up to approximately 15% of wild type levels, whereas no complexes with oxygen evolution activity could be detected in the glutamine mutant. PSII supercomplexes isolated from the tyrosine and methionine mutants were as active as wild type in terms of light-saturated rates of oxygen evolution but in contrast to wild type contained no bound heme despite the presence of the alpha subunit. Oxygen evolution in the tyrosine and methionine mutants was, however, more sensitive to photoinactivation than the WT. Overall, these data establish unambiguously that a redox role for the heme of cytochrome b(559) is not required for photosynthetic oxygen evolution. Instead, our data provide new evidence of a role for cytochrome b(559) in the protection of the photosystem two complex in vivo.

Amino acid residues that modulate the properties of tyrosine Y(Z) and the manganese cluster in the water oxidizing complex of photosystem II

The catalytic site for photosynthetic water oxidation is embedded in a protein matrix consisting of nearly 30 different polypeptides. Residues from several of these polypeptides modulate the properties of the tetrameric Mn cluster and the redox-active tyrosine residue, Y(Z), that are located at the catalytic site. However, most or all of the residues that interact directly with Y(Z) and the Mn cluster appear to be contributed by the D1 polypeptide. This review summarizes our knowledge of the environments of Y(Z) and the Mn cluster as obtained from the introduction of site-directed, deletion, and other mutations into the photosystem II polypeptides of the cyanobacterium Synechocystis sp. PCC 6803 and the green alga Chlamydomonas reinhardtii.

Antioxidant responses of cucumber (Cucumis sativus) to photoinhibition and oxidative stress induced by norflurazon under high and low PPFDs

Photooxidative damage is exacerbated by norflurazon (NF), which blocks carotenoid biosynthesis. This study examined the influence of photosynthetic photon flux density (PPFD) on the overall responses of both non-enzymatic and enzymatic antioxidants to NF-caused oxidative damage in leaves of cucumber (Cucumis sativus). Seven-day-old cucumber plants were exposed to NF under either low PPFD (30 µmol m(-2) s(-1)) or high PPFD (300 µmol m(-2) s(-1)) for 3 days. The NF plants exposed at high PPFD had lower levels of F(v)/F(m) ratio, quantum yield of electron transport, and 33-kDa protein of photosystem II as compared with the NF plants at low PPFD. In the NF plants, there was a reduction in total chlorophylls and carotenoids except newly formed zeaxanthin in either PPFD. The NF plants at high PPFD resulted in less level of photochemical quenching, q(P), and Stern-Volmer quenching, NPQ, than those of the plants at low PPFD, whereas both plants had similar level of non-photochemical quenching coefficient, q(N). However, the level of PPFD did not significantly affect the NF-caused induction of antioxidant enzymes including peroxidase, superoxide dismutase, glutathione reductase, and ascorbate peroxidase.

The chloroplast-encoded alpha subunit of cytochrome b-559 is required for assembly of the photosystem two complex in both the light and the dark in Chlamydomonas reinhardtii

The role of cytochrome b-559 in the photosystem two (PSII) complex has been investigated through the construction of a psbE null mutant by transformation of the chloroplast genome of the green alga Chlamydomonas reinhardtii. No PSII activity could be detected in this mutant either in oxygen evolution assays or by analysis of variable chlorophyll fluorescence. Immunoblotting experiments showed that the absence of PSII activity in the mutant was due to the loss of the PSII complex in both light-grown and dark-grown cultures. In contrast, the photosystem one reaction center polypeptide, PsaA, was present at wild-type levels in the mutant. RNA gel blot assays confirmed that the transcript levels for the psbA, psbD, and psbF genes were unaffected by disruption of the psbE gene, suggesting a post-transcriptional effect on their expression. Pulse-labeling experiments showed that either synthesis of PSII subunits was impaired in the psbE null mutant or there was extremely rapid degradation of newly synthesized subunits. Interestingly, the PsbE and PsbF subunits accumulated to wild-type levels in a psbA deletion mutant of C. reinhardtii, FuD7, which fails to synthesize D1 and assemble PSII. Our results provide evidence for a role for cytochrome b-559 in the early steps of assembly of the PSII complex, possibly as a redox-controlled nucleation factor that determines the level of PSII within the thylakoid membrane.

D1 polypeptide degradation may regulate psbA gene expression at transcriptional and translational levels in Synechocystis sp. PCC 6803

Light has been suggested to regulate both synthesis and degradation of the Photosystem II (PS II) reaction centre polypeptide D1, encoded by the psbA gene. The modified degradation rate of the D1 polypeptide in site-directed Synechocystis sp PCC 6803 D1 mutants CA1 [del(E242-E244);Q241H], E243K and E229D has provided a tool to determine whether the rate of D1 polypeptide synthesis is directly regulated by light-intensity-related factors or by a control mechanism mediated by light-dependent degradation of the D1 polypeptide. In vivo accumulation of [(35)S] methionine into the D1 polypeptide was found to correlate with D1 polypeptide degradation rather than with incident irradiance. This suggests that the degradation rate of the D1 polypeptide regulates its own synthesis at translational level. Furthermore, several fold differences in the psbA mRNA levels were measured between D1 mutant strains, indicating that the psbA gene transcription is not solely under light control.

Restoration of the high potential form of cytochrome b-559 through the photoreactivation of Tris-inactivated oxygen-evolving center

Restoration of a high potential (HP) form of cytochrome b-559 (Cyt b-559) from a low potential (LP) form was the primary process in the reconstitution of O2-evolving center during the photoreactivation of Tris-inactivated chloroplasts. In normal chloroplasts, about 0.5 to 0.7 mol of Cyt b-559 was present in the HP form per 400 chlorophyll molecules. However, the HP form was converted to the LP form when the O2-evolving center was inactivated by 0.8 M alkaline Tris-washing (pH 9.1). The inactivation was reversible and both the Cyt b-559 HP form and the O2-evolving activity were restored by incubating the inactivated chloroplasts with weak light, Mn(2+), Ca(2+) and an electron donor (photoreactivation). The recovery of the HP form preceded the recovery of O2-evolving activity. 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) did not inhibit the recovery of the HP form. Thus, the recovery of Cyt b-559 HP form was the primary reaction in the photoreactivation, which was stimulated by the light-induced redox reaction of the PS-II core center.

Topography of the heme prosthetic group of cytochrome b-559 in the photosystem II reaction center

The topography of the heme prosthetic group of cytochrome b-559 of the photosystem II reaction center was determined from measurement of the orientation of its alpha- and beta-polypeptides in thylakoid membranes of spinach chloroplasts and in osmotically disrupted cells of the cyanobacterium Synechocystis sp. PCC 6803. The accessibility to trypsin proteolysis of an epitope located near the solvent-exposed N-terminus of the beta-subunit was compared to that of the alpha-subunit, whose N- and C-termini had previously been localized from the trypsinolysis pattern to the stromal and lumenal sides of spinach thylakoid membranes, respectively [Tae et al. (1988) Biochemistry 27, 9075-9080; Vallon et al. (1989) Biochim. Biophys. Acta 975, 132-141]. The N-terminal epitope of the cyanobacterial beta-subunit was modified by introducing a tridecapeptide epitope, previously found to be immunoreactive, from the C-terminal region of the spinach chloroplast alpha-subunit. This epitope had no homology with the cyanobacterial alpha-subunit. The cells with the hybrid beta-subunit retained full photosynthetic activity. The intactness of membranes from osmotically shocked cyanobacteria was tested by trypsin inaccessibility to (a) the alpha-subunit C-terminus and (b) the manganese-stabilizing protein (MSP) of the oxygen-evolving complex that is on the lumenal side of the membrane. The loss after trypsinolysis of most of the beta-subunit immunoreactivity, under conditions where (i) the alpha-subunit was cleaved near the N-terminus in both spinach thylakoids and osmotically shocked cyanobacterial membranes and (ii) the MSP protein in cyanobacteria was not disrupted, implied that the orientation of the beta-subunit was parallel to that of the alpha-subunit in both kinds of membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes of amino acid sequence in PEST-like area and QEEET motif affect degradation rate of D1 polypeptide in photosystem II

The degradation rate of the D1 polypeptide was measured in three Synechocystis PCC 6803 mutants in vivo. Mutations were introduced into a putative cleavage area of the D1 polypeptide (QEEET motif) and into the PEST-like area. PEST sequences are often found in proteins with a high turnover rate. The QEEET-motif mutants are CA1 [delta (E242-E244);Q241H] and E243K, and the third mutation, E229D, was directed to the PEST-like area. During high-light illumination (1500 mumol photons m-2 s-1) that induced photoinhibition of photosystem II (PSII), the half-life time of the D1 polypeptide in mutant E229D (t 1/2 = 35 min) was about twice as long as in AR (control strain) cells (t 1/2 = 19 min). In growth light (40 mumol photons m-2 s-1), the degradation rate of the D1 polypeptide in E229D and AR strains was the same (t 1/2 approximately 5 h). In growth light the D1 polypeptide was degraded faster in both QEEET-motif mutants than in the AR strain, but in photoinhibitory light the degradation rates were similar. According to these results, the highly conservative QEEET motif as such is not required for the proteolytic cut of the D1 polypeptide, but it does affect the rate of degradation. No simple correlation existed between the degradation rate of the D1 polypeptide and the susceptibility of PSII to photoinhibition in mutant and AR cells under our experimental conditions.

Characterization of the chloroplast cytochrome b6f complex as a structural and functional dimer

Size analysis of the cytochrome b6f complex by FPLC Superose-12 chromatography and Blue Native PAGE indicated a predominantly dimeric component with M(r) = (1.9-2.5) x 10(5). The true dimer molecular weight including bound lipid, but not detergent, was estimated to be 2.3 x 10(5). Size and shape analysis by negative-stain single-particle electron microscopy indicated that the preparation of dimeric complexes contains a major population that has a protein cross section 40% larger than the monomer, binds more negative stain, and has a geometry with a distinct 2-fold axis of symmetry compared to the monomeric complex. The dimeric species is more stable at higher ionic strength with respect to conversion to the monomeric species. SDS-PAGE of monomer and dimer preparations indicated that both contain the four major polypeptides in approximately equal stoichiometry and also contain the petG M(r) 4000 subunit. One bound chlorophyll a per monomer, part of the bound lipid, is present in monomer and dimer. The in vitro electron-transport activity (decyl-PQH2-->PC-ferricyanide) of the separated dimer was comparable to that of the isolated b6f complex and was 4-5-fold greater than that of the monomer preparation, whose activity could be attributed to residual dimer. No difference in the properties of the dimer and monomer was detected by SDS-PAGE or redox difference spectrophotometry that could account for the difference in activities. However, the concentration of the Rieske [2Fe-2S] center was found by EPR analysis of the gy = 1.90 signal to be lower in the monomer fraction by a factor of 3.5 relative to the dimer.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of cytochrome b-559 content in photosystem II complexes from spinach and Synechocystis species PCC 6803

Cytochrome b-559 is an integral component of photosystem II complexes from both plants and cyanobacteria. However, the number of cytochrome b-559 associated with the photosystem II reaction center has been the subject of controversy. Some studies have concluded that there is one heme equivalent of cytochrome b-559 per reaction center, some studies have found two, and some studies have reported intermediate values. Most of the previous experiments have used only one method to quantitate the antenna size of the preparation. In this study, we compare the cytochrome b-559 content in a cyanobacterial and a plant photosystem II preparation. The plant preparation is derived from spinach, and previous work has shown that it has an antenna size of approximately 100 chlorophylls [MacDonald, G. M., & Barry, B. A. (1992) Biochemistry 31, 9848-9856]. The cyanobacterial preparation is from Synechocystis sp. PCC 6803, and previous work has shown that it has an antenna size of approximately 60 chlorophylls [Noren, G. H., Boerner, R. J., & Barry, B. A. (1991) Biochemistry 30, 3943-3950]. Both preparations are isolated through the use of ion-exchange chromatography, and both preparations are monodisperse in the same nonionic detergent. In our comparative study, we quantitate antenna size by three different methods. Our work shows that, depending on the method used to estimate antenna size, the oxygen-evolving spinach photosystem II preparation contains 0.82-1.0 cytochrome b-559 per reaction center, while the oxygen-evolving cyanobacterial preparation contains 1.5-2.1 cytochrome b-559 per reaction center.

The enigmatic cytochrome b-559 of oxygenic photosynthesis

The ubiquitous and obligatory association of cytochrome b-559 with the photosystem II reaction center of oxygenic photosynthesis is a conundrum since it seems not to have a function in the primary electron transport pathway of oxygen evolution. A model for the cytochrome structure that satisfies the cis-positive rule for membrane protein assembly consists of two short, non-identical hydrophobic membrane-spanning polypeptides (α and β), each containing a single histidine residue, as ligands for the bridging heme prosthetic group that is on the side of the membrane opposite to the water splitting apparatus. The ability of the heterodimer, but not the single α-subunit, to satisfy the cis-positive rule implies that the cytochrome inserts into the membrane as a heterodimer, with some evidence implicating it as the first membrane inserted unit of the assembling reaction center. The very positive redox potential of the cytochrome can be explained by a position for the heme in a hydrophobic niche near the stromal aqueous interface where it is also influenced by the large positive dipole potential of the parallel α-helices of the cytochrome. The requirement for the cytochrome in oxygenic photosynthesis may be a consequence of the presence of the strongly oxidizing reaction center needed for H₂ O-splitting. This may lead to the need, under conditions of stress or plastid development, for an alternate source of electrons when the H₂ O-splitting system is not operative as a source of reductant for the reaction center.

Role of the carboxy terminus of polypeptide D1 in the assembly of a functional water-oxidizing manganese cluster in photosystem II of the cyanobacterium Synechocystis sp. PCC 6803: assembly requires a free carboxyl group at C-terminal position 344

The D1 polypeptide of the photosystem II (PSII) reaction center is synthesized as a precursor polypeptide which is posttranslationally processed at the carboxy terminus. It has been shown in spinach that such processing removes nine amino acids, leaving Ala344 as the C-terminal residue [Takahashi, M., Shiraishi, T., & Asada, K. (1988) FEBS Lett. 240, 6-8; Takahashi, Y., Nakane, H., Kojima, H., & Satoh, K. (1990) Plant Cell Physiol. 31, 273-280]. We show here that processing on the carboxy side of Ala344 also occurs in the cyanobacterium Synechocystis 6803, resulting in the removal of 16 amino acids. By constructing a deletion strain of Synechocystis 6803 that lacks the three copies of the psbA gene encoding D1, we have developed a system for generating psbA mutants. Using this system, we have constructed mutants of Synechocystis 6803 that are modified in the region of the C-terminus of the D1 polypeptide. Characterization of these mutants has revealed that (1) processing of the D1 polypeptide is blocked when the residue after the cleavage site is changed from serine to proline (mutant Ser345Pro) with the result that the manganese cluster is unable to assemble correctly; (2) the C-terminal extension of 16 amino acid residues can be deleted with little consequence either for insertion of D1 into the thylakoid membrane or for assembly of D1 into a fully active PSII complex; (3) removal of only one more residue (mutant Ala344stop) results in a loss of assembly of the manganese cluster; and (4) the ability of detergent-solubilized PSII core complexes (lacking the manganese cluster) to bind and oxidize exogenous Mn2+ by the secondary donor, Z+, is largely unaffected in the processing mutants (the Ser345Pro mutant of Synechocystis 6803 and the LF-1 mutant of Scenedesmus obliquus) and the truncation mutant Ala344stop. Our results are consistent with a role for processing in regulating the assembly of the photosynthetic manganese cluster and a role for the free carboxy terminus of the mature D1 polypeptide in the ligation of one or more manganese ions of the cluster.