Genetically engineered mutant of the cyanobacterium Synechocystis 6803 lacks the photosystem II chlorophyll-binding protein CP-47

Electrochemically Driven Photosynthetic Electron Transport in Cyanobacteria Lacking Photosystem II

Light-activated photosystem II (PSII) carries out the critical step of splitting water in photosynthesis. However, PSII is susceptible to light-induced damage. Here, results are presented from a novel microbial electro-photosynthetic system (MEPS) that uses redox mediators in conjunction with an electrode to drive electron transport in live Synechocystis (ΔpsbB) cells lacking PSII. MEPS-generated, light-dependent current increased with light intensity up to 2050 μmol photons m^-2 s^-1, which yielded a delivery rate of 113 μmol electrons h^-1 mg-chl^-1 and an average current density of 150 A m^-2 s^-1 mg-chl^-1. P700⁺ re-reduction kinetics demonstrated that initial rates exceeded wildtype PSII-driven electron delivery. The electron delivery occurs ahead of the cytochrome b₆f complex to enable both NADPH and ATP production. This work demonstrates an electrochemical system that can drive photosynthetic electron transport, provides a platform for photosynthetic foundational studies, and has the potential for improving photosynthetic performance at high light intensities.

The international journeys and aliases of Synechococcus elongatus

This perspective provides a historical account of the isolation and nomenclature of the cyanobacterial strains currently known as Synechococcus elongatus. The story focuses on an isolate from the San Francisco Bay area of California (Pasteur Culture Collection PCC 7942) that has, for decades, been the genetic model for this species, and its close relative isolated from Waller Creek in Texas (PCC 6301, also known as the University of Texas at Austin Culture Collection of Algae UTEX 625). Until recently, these strains have been the only representatives of the species. A new wild isolate, UTEX 3055, is distinctly different from the prior reference strains. S. elongatus strains have been widely used by labs around the world to discover fundamental cellular processes and to engineer cyanobacteria to generate useful products. The review clarifies relationships among strains that carry different names, and explains how names that appear in the literature have changed over the years.

The stress-induced SCP/HLIP family of small light-harvesting-like proteins (ScpABCDE) protects Photosystem II from photoinhibitory damages in the cyanobacterium Synechocystis sp. PCC 6803

Small CAB-like proteins (SCPs) are single-helix light-harvesting-like proteins found in all organisms performing oxygenic photosynthesis. We investigated the effect of growth in moderate salt stress on these stress-induced proteins in the cyanobacterium Synechocystis sp. PCC 6803 depleted of Photosystem I (PSI), which expresses SCPs constitutively, and compared these cells with a PSI-less/ScpABCDE^- mutant. SCPs, by stabilizing chlorophyll-binding proteins and Photosystem II (PSII) assembly, protect PSII from photoinhibitory damages, and in their absence electrons accumulate and will lead to ROS formation. The presence of 0.2 M NaCl in the growth medium increased the respiratory activity and other PSII electron sinks in the PSI-less/ScpABCDE^- strain. We postulate that this salt-induced effect consumes the excess of PSII-generated electrons, reduces the pressure of the electron transport chain, and thereby prevents ¹O₂ production.

Generation of Marked and Markerless Mutants in Model Cyanobacterial Species

Cyanobacteria are ecologically important organisms and potential platforms for production of biofuels and useful industrial products. Genetic manipulation of cyanobacteria, especially model organisms such as Synechocystis sp. PCC6803 and Synechococcus sp. PCC7002, is a key tool for both basic and applied research. Generation of unmarked mutants, whereby chromosomal alterations are introduced into a strain via insertion of an antibiotic resistance cassette (a manipulatable fragment of DNA containing one or more genes), followed by subsequent removal of this cassette using a negative selectable marker, is a particularly powerful technique. Unmarked mutants can be repeatedly genetically manipulated, allowing as many alterations to be introduced into a strain as desired. In addition, the absence of genes encoding antibiotic resistance proteins in the mutated strain is desirable, as it avoids the possibility of 'escape' of antibiotic resistant organisms into the environment. However, detailed methods for repeated rounds of genetic manipulation of cyanobacteria are not well described in the scientific literature. Here we provide a comprehensive description of this technique, which we have successfully used to generate mutants with multiple deletions, single point mutations within a gene of interest and insertion of novel gene cassettes.

A bioelectrochemical approach to characterize extracellular electron transfer by Synechocystis sp. PCC6803

Biophotovoltaic devices employ photosynthetic organisms at the anode of a microbial fuel cell to generate electrical power. Although a range of cyanobacteria and algae have been shown to generate photocurrent in devices of a multitude of architectures, mechanistic understanding of extracellular electron transfer by phototrophs remains minimal. Here we describe a mediatorless bioelectrochemical device to measure the electrogenic output of a planktonically grown cyanobacterium, Synechocystis sp. PCC6803. Light dependent production of current is measured, and its magnitude is shown to scale with microbial cell concentration and light intensity. Bioelectrochemical characterization of a Synechocystis mutant lacking Photosystem II demonstrates conclusively that production of the majority of photocurrent requires a functional water splitting aparatus and electrons are likely ultimately derived from water. This shows the potential of the device to rapidly and quantitatively characterize photocurrent production by genetically modified strains, an approach that can be used in future studies to delineate the mechanisms of cyanobacterial extracellular electron transport.

A single vector-based strategy for marker-less gene replacement in Synechocystis sp. PCC 6803

BACKGROUND

The cyanobacterium Synechocystis sp. PCC 6803 is widely used for research on photosynthesis and circadian rhythms, and also finds application in sustainable biotechnologies. Synechocystis is naturally transformable and undergoes homologous recombination, which enables the development of a variety of tools for genetic and genomic manipulations. To generate multiple gene deletions and/or replacements, marker-less manipulation methods based on counter-selection are generally employed. Currently available methods require two transformation steps with different DNA plasmids.

RESULTS

In this study, we present a marker-less gene deletion and replacement strategy in Synechocystis sp. PCC 6803 which needs only a single transformation step. The method utilizes an nptI-sacB double selection cassette and exploits the ability of the cyanobacterium to undergo two successive genomic recombination events via double and single crossing-over upon application of appropriate selective procedures.

CONCLUSIONS

By reducing the number of cloning steps, this strategy will facilitate gene manipulation, gain-of-function studies, and automated screening of mutants.

Evidence for a role of chloroplastic m-type thioredoxins in the biogenesis of photosystem II in Arabidopsis

Chloroplastic m-type thioredoxins (TRX m) are essential redox regulators in the light regulation of photosynthetic metabolism. However, recent genetic studies have revealed novel functions for TRX m in meristem development, chloroplast morphology, cyclic electron flow, and tetrapyrrole synthesis. The focus of this study is on the putative role of TRX m1, TRX m2, and TRX m4 in the biogenesis of the photosynthetic apparatus in Arabidopsis (Arabidopsis thaliana). To that end, we investigated the impact of single, double, and triple TRX m deficiency on chloroplast development and the accumulation of thylakoid protein complexes. Intriguingly, only inactivation of three TRX m genes led to pale-green leaves and specifically reduced stability of the photosystem II (PSII) complex, implying functional redundancy between three TRX m isoforms. In addition, plants silenced for three TRX m genes displayed elevated levels of reactive oxygen species, which in turn interrupted the transcription of photosynthesis-related nuclear genes but not the expression of chloroplast-encoded PSII core proteins. To dissect the function of TRX m in PSII biogenesis, we showed that TRX m1, TRX m2, and TRX m4 interact physically with minor PSII assembly intermediates as well as with PSII core subunits D1, D2, and CP47. Furthermore, silencing three TRX m genes disrupted the redox status of intermolecular disulfide bonds in PSII core proteins, most notably resulting in elevated accumulation of oxidized CP47 oligomers. Taken together, our results suggest an important role for TRX m1, TRX m2, and TRX m4 proteins in the biogenesis of PSII, and they appear to assist the assembly of CP47 into PSII.

Energy biotechnology with cyanobacteria

The world's future energy demand calls for a sustainable alternative for the use of fossil fuels, to restrict further global warming. Harvesting solar energy via photosynthesis is one of Nature's remarkable achievements. Existing technologies exploit this process for energy 'production' via processing of, for example, part of plant biomass into ethanol, and of algal biomass into biodiesel. Fortifying photosynthetic organisms with the ability to produce biofuels directly would bypass the need to synthesize all the complex chemicals of 'biomass'. A promising way to achieve this is to redirect cyanobacterial intermediary metabolism by channeling (Calvin cycle) intermediates into fermentative metabolic pathways. This review describes this approach via the biosynthesis of fermentation end products, like alcohols and hydrogen, driven by solar energy, from water (and CO2).

Spectral properties of the CP43-deletion mutant of Synechocystis sp. PCC 6803

Spectral properties, particularly fluorescence spectra and their time-dependent behavior, were investigated for a mutant of the cyanobacterium Synechocystis sp. PCC 6803 lacking the 43 kDa chlorophyll-protein (CP43, PsbC). Lack of CP43 was confirmed by a size shift of the corresponding gene and by Western blotting. The CP43-deletion mutant grown under heterotrophic conditions accumulated a small amount of photosystem (PS) II, but virtually no PS II fluorescence was observed. A 686-nm fluorescence band was clearly observed by phycocyanin excitation, coming from the terminal pigments of phycobilisomes. In contrast, no PS I fluorescence was detected by phycocyanin excitation when accumulation of PS II components was not proved by a fluorescence excitation spectrum, indicating that energy transfer to PS I chlorophyll a was mediated by PS II chlorophyll a. Direct connection of phycobilisomes with PS I was not suggested. Based on these fluorescence properties, the energy flow in the CP43-deletion mutant cells is discussed.

Digalactosyldiacylglycerol is required for stabilization of the oxygen-evolving complex in photosystem II

The galactolipid digalactosyldiacylglycerol (DGDG) is present in the thylakoid membranes of oxygenic photosynthetic organisms such as higher plants and cyanobacteria. Recent x-ray crystallographic analysis of protein-cofactor supercomplexes in thylakoid membranes revealed that DGDG molecules are present in the photosystem II (PSII) complex (four molecules per monomer), suggesting that DGDG molecules play important roles in folding and assembly of subunits in the PSII complex. However, the specific role of DGDG in PSII has not been fully clarified. In this study, we identified the dgdA gene (slr1508, a ycf82 homolog) of Synechocystis sp. PCC6803 that presumably encodes a DGDG synthase involved in the biosynthesis of DGDG by comparison of genomic sequence data. Disruption of the dgdA gene resulted in a mutant defective in DGDG synthesis. Despite the lack of DGDG, the mutant cells grew as rapidly as the wild-type cells, indicating that DGDG is not essential for growth in Synechocystis. However, we found that oxygen-evolving activity of PSII was significantly decreased in the mutant. Analyses of the PSII complex purified from the mutant cells indicated that the extrinsic proteins PsbU, PsbV, and PsbO, which stabilize the oxygen-evolving complex, were substantially dissociated from the PSII complex. In addition, we found that heat susceptibility but not dark-induced inactivation of oxygen-evolving activity was notably increased in the mutant cells in comparison to the wild-type cells, suggesting that the PsbU subunit is dissociated from the PSII complex even in vivo. These results demonstrate that DGDG plays important roles in PSII through the binding of extrinsic proteins required for stabilization of the oxygen-evolving complex.

Effects of the lack of phosphatidylglycerol on the donor side of photosystem II

Our previous studies with the pgsA mutant of the cyanobacterium Synechocystis sp. PCC6803 (hereafter termed pgsA mutant), which is defective for the biosynthesis of phosphatidylglycerol (PG), revealed an important role for PG in the electron acceptor side of photosystem II (PSII), especially in the electron transport between plastoquinones Q(A) and Q(B). This study now shows that PG also plays an important role in the electron donor side of PSII, namely, the oxygen-evolving system. Analyses of purified PSII complexes indicated that PSII from PG-depleted pgsA mutant cells sustained only approximately 50% of the oxygen-evolving activity compared to wild-type cells. Dissociation of the extrinsic proteins PsbO, PsbV, and PsbU, which are required for stabilization of the manganese (Mn) cluster, followed by the release of a Mn atom, was observed in PSII of the PG-depleted mutant cells. The released PsbO rebound to PSII when PG was added back to the PG-depleted mutant cells, even when de novo protein synthesis was inhibited. Changes in photosynthetic activity of the PG-depleted pgsA mutant cells induced by heat treatment or dark incubation resembled those of DeltapsbO, DeltapsbV, and DeltapsbU mutant cells. These results suggest that PG plays an important role in binding extrinsic proteins required for sustaining a functional Mn cluster on the donor side of PSII.

Gene-targeted and site-directed mutagenesis of photosynthesis genes in cyanobacteria

This historical minireview traces the development and application of methods for gene-targeted and site-directed mutagenesis of photosynthesis genes in cyanobacteria (mainly Synechocystis sp. PCC 6803). This approach allowed important data to be obtained on the structure and function of Photosystem I and Photosystem II complexes. I describe some of the major contributions of molecular genetics and subsequent mutant analysis in the 1980s and early 1990s that led to substantial advances in our knowledge of basic principles regarding the organization of the photosynthetic apparatus. This molecular-genetic research on cyanobacteria has initiated a fresh wave of photosynthesis research and created a solid foundation for rapid progress at the threshold of the twenty-first century.

Changes in the composition of the photosynthetic apparatus in the galactolipid-deficient dgd1 mutant of Arabidopsis thaliana

The glycerolipid digalactosyl diacylglycerol (DGDG) is exclusively associated with photosynthetic membranes and thus may play a role in the proper assembly and maintenance of the photosynthetic apparatus. Here we employ a genetic approach based on the dgd1 mutant of Arabidopsis thaliana to investigate the function of DGDG in thylakoid membranes. The primary defect in the genetically well-characterized dgd1 mutant resulted in a 90% reduction of the DGDG content. The mutant showed a decreased photosystem II (PSII) to photosystem I ratio. In vivo room- and low-temperature (77 K) chlorophyll fluorescence measurements with thylakoid preparations are in agreement with a drastically altered excitation energy allocation to the reaction centers. Quantification of pigment-binding apoproteins and pigments supports an altered stoichiometry of individual pigment-protein complexes in the mutant. Most strikingly, an increase in the amount of peripheral light-harvesting complexes of PSII relative to the inner antenna complexes and the PSII reaction center/core complexes was observed. Regardless of the severe alterations in thylakoid organization, photosynthetic oxygen evolution was virtually not compromised in dgd1 mutant leaves.

A null mutant of Synechococcus sp. PCC7942 deficient in the sulfolipid sulfoquinovosyl diacylglycerol

The sulfolipid 6-sulfo-alpha-D-quinovosyldiacylglycerol is associated with the thylakoid membranes of many photosynthetic organisms. Previously, genes involved in sulfolipid biosynthesis have been characterized only in the purple bacterium Rhodobacter sphaeroides. Unlike plants and cyanobacteria, photosynthesis in this bacterium is anoxygenic due to the lack of a water splitting photosystem II. To test the function of sulfolipid in an organism with oxygenic photosynthesis, we isolated and inactivated a sulfolipid gene of the cyanobacterium Synechococcus sp. PCC7942. Extensive analysis of the sulfolipid-deficient null mutant revealed subtle changes in photosynthesis related biochemistry of O2. In addition, a slight increase in the variable room temperature chlorophyll fluorescence yield was observed. Regardless of these changes, it seems unlikely that sulfolipid is an essential constituent of a functional competent water oxidase or the core antenna complex of photosystem II. However, reduced growth of the mutant under phosphate-limiting conditions supports the hypothesis that sulfolipid acts as a surrogate for anionic phospholipids under phosphate-limiting growth conditions.

Light-dependent chlorophyll a biosynthesis upon chlL deletion in wild-type and photosystem I-less strains of the cyanobacterium Synechocystis sp. PCC 6803

Part of the chlL gene encoding a component involved in light-independent protochlorophyllide reduction was deleted in wild type and in a photosystem I-less strain of Synechocystis sp. PCC 6803. In resulting mutants, chlorophyll biosynthesis was fully light-dependent. When these mutants were propagated under light-activated heterotrophic growth conditions (in darkness except for 15 min of weak light a day) for several weeks, essentially no chlorophyll was detectable but protochlorophyllide accumulated. Upon return of the chlL- mutant cultures to continuous light, within the first 6 h chlorophyll was synthesized at the expense of protochlorophyllide at a rate independent of the presence of photosystem I. Chlorophyll biosynthesized during this time gave rise to a 685 nm fluorescence emission peak at 77 K in intact cells. This peak most likely originates from a component different from those known to be directly associated with photosystems II and I. Development of 695 and 725 nm peaks (indicative of intact photosystem II and photosystem I, respectively) required longer exposures to light. After 6 h of greening, the rate of chlorophyll synthesis slowed as protochlorophyllide was depleted. In the chlL- strain, greening occurred at the same rate at two different light intensities (5 and 50 microE m-2 s-1), indicating that also at low light intensity the amount of light is not rate-limiting for protochlorophyllide reduction. Thus, in this system the rate of chlorophyll biosynthesis is limited neither by biosynthesis of photosystems nor by the light-dependent protochlorophyllide reduction. We suggest the presence of a chlorophyll-binding 'chelator' protein (with 77 K fluorescence emission at 685 nm) that binds newly synthesized chlorophyll and that provides chlorophyll for newly synthesized photosynthetic reaction centers and antennae.

Loss of chloroplast transcripts for proteins associated with photosystem II: an early event during heat-bleaching in Euglena gracilis

A shift in the ratio of chlorophyll (Chl) a and Chl b is an early indicator of heat bleaching in Euglena gracilis. This observation prompted us to consider whether or not changes in steady-state levels of chloroplast transcripts and in transcriptional activity could limit the synthesis of Chl a-binding proteins in bleaching plastids. We found that the mature transcripts for CP47 and CP43, the Chl a-binding apoproteins of the proximal antenna of photosystem II, decline sharply very early during bleaching. Our study also shows that transcription of psbB and psbC, the chloroplast genes encoding CP47 and CP43, remains essentially unchanged during the same interval. We conclude that posttranscriptional events, such as mRNA stability, could play a major role in initiating an irreversible loss of chloroplast function in Euglena at a moderately elevated temperature. Lack of these transcripts would eventually impair the assembly of photosystem II in thylakoids.

Functional characterization of mutant strains of the cyanobacterium Synechocystis sp. PCC 6803 lacking short domains within the large, lumen-exposed loop of the chlorophyll protein CP47 in photosystem II

Several autotrophic mutant strains of Synechocystis sp. PCC 6803 carrying short deletions or a single-site mutation within the large, lumen-exposed loop (loop E) of the chlorophyll a-binding photosystem II core protein, CP47, are analyzed for their functional properties by measuring the flash-induced pattern of thermoluminescence, oxygen yield, and fluorescence quantum yield. A physiological and biochemical characterization of these mutant strains has been given in two previous reports [Eaton-Rye, J.J., & Vermaas, W.F.J. (1991) Plant Mol. Biol. 17, 1165-1177; Haag, E., Eaton-Rye, J.J., Renger, G., & Vermaas, S. F.J. (1993) Biochemistry 32, 4444-4454]. The results of the present study show that deletion of charged and conserved amino acids in a region roughly located between residues 370 and 390 decreases the binding affinity of the extrinsic PS II-O protein to photosystem II. Marked differences with PSII-O deletion mutants are observed with respect to Ca2+ requirement and the flash-induced pattern of oxygen evolution. Under conditions where a sufficient light activation is provided, the psbB mutants assayed in this study reveal normal S-state parameters and lifetimes. The results bear two basic implications: (i) the manganese involved in water oxidation can still be bound in a functionally normal or only slightly distorted manner, and (ii) the binding of the extrinsic PS II-O protein to photosystem II is impaired in mutants carrying a deletion in the domain between residues 370 and 390, but the presence of the PS II-O protein is still of functional relevance for the PS II complex, e.g., for maintenance of a high-affinity binding site for Ca2+ and/or involvement during the process of photoactivation.

Physical genome map of the unicellular cyanobacterium Synechococcus sp. strain PCC 7002

A physical restriction map of the genome of the cyanobacterium Synechococcus sp. strain PCC 7002 was assembled from AscI, NotI, SalI, and SfiI digests of intact genomic DNA separated on a contour-clamped homogeneous electric field pulsed-field gel electrophoresis system. An average genome size of 2.7 x 10(6) bp was calculated from 21 NotI, 37 SalI, or 27 SfiI fragments obtained by the digestions. The genomic map was assembled by using three different strategies: linking clone analysis, pulsed-field fragment hybridization, and individual clone hybridization to singly and doubly restriction-digested large DNA fragments. The relative positions of 21 genes or operons were determined, and these data suggest that the gene order is not highly conserved between Synechococcus sp. strain PCC 7002 and Anabaena sp. strain PCC 7120.

Cloning and sequencing of mutantpsbB genes of the cyanobacteriumSynechocystis PCC 6803

Ten strains from a collection of mutants ofSynechocystis 6803 defective in Photosystem II (PS II) function were transformed with chromosomal DNA of wild-type and mutant cells. Cross hybridization data allowed to identify four groups of PS II-mutants. Highly efficient transformation was observed between different mutant groups, but not within the groups. Restoration of photosynthetic activity of the mutant cells was also achieved by transformation with different parts of a 5.6 kbBam HI fragment of wild typeSynechocystis DNA containing thepsbB gene. Each group of mutants was transformed to photoautotrophic growth by specific subfragments of thepsbB gene. DNA fragments of four selected mutant strains hybridizing with thepsbB gene were isolated and sequenced. The mutations were identified as a single nucleotide insertion or substitution leading to stop codon formation in two of the mutants, as a deletion of 12 nucleotides, or as a nucleotide substitution resulting in an amino acid substitution in the other two mutants. Deletion of 12 nucleotides in mutant strain PMB1 and stop codon formation in strain NF16 affect membrane-spanning regions of the gene product, the CP 47 protein.

Restoration of the wild-type locus in an RuBP carboxylase/oxygenase mutant of Synechocystis PCC 6803 via targeted gene recombination

The interaction between homologous DNA sequences, distant from each other in the chromosome, was examined in the cyanobacterium Synechocystis PCC 6803. Most of the rbcL gene encoding the large subunit of ribulose bisphosphate carboxylase/oxygenase (Rubisco) was duplicated in the genome by a targeted insertion of a 3'-truncated gene copy into the psb A-I locus. Both rbcL genes, in the psb A-I region and at the rbc locus, were non-functional; The former due to the 3' truncation, and the latter due to a deletion in the 5'-region (creating a 5' truncation) and a mutation associated with an insertion of the Rhodospirillum rubrum rbc gene, yielding a high-CO2-requiring mutant ('cyanorubrum'). The 3' and the 5' truncated rbcL genes were linked to chloramphenicol and kanamycin resistance markers, respectively. Decreasing the kanamycin selective pressure concomitantly with exposure of the double resistance mutant to air, resulted in air-growing colonies. Analysis of their genomes, Rubisco proteins, and their ultrastructure revealed: 1) Reconstitution of a full-length cyanobacterial rbcL gene at the rbc locus; 2) simultaneous synthesis of the cyanobacterial (L8S8) and R. rubrum (L2) enzymes in meroploids containing both mutated and reconstituted rbcL genes; 3) reappearance of carboxysomes. Our results indicate extensive recombinatorial interactions between the homologous sequences at both loci leading to reconstitution of the cyanobacterial rbcL gene.

Changes in the photosynthetic apparatus in the cyanobacterium Synechocystis sp. PCC 6714 following light-to-dark and dark-to-light transitions

The photosynthetic apparatus of Synechocystis sp. PCC 6714 cells grown chemoheterotrophically (dark with glucose as a carbon source) and photoautotrophically (light in a mineral medium) were compared. Dark-grown cells show a decrease in phycocyanin content and an even greater decrease in chlorophyll content with respect to light-grown cells. Analysis of fluorescence emission spectra at 77 K and at 20 °C, of dark- and light-grown cells, and of phycobilisomes isolated from both types of cells, indicated that in darkness the phycobiliproteins were assembled in functional phycobilisomes (PBS). The dark synthesized PBS, however, were unable to transfer their excitation energy to PS II chlorophyll. Upon illumination of dark-grown cells, recovery of photosynthetic activity, pigment content and energy transfer between PBS and PS II was achieved in 24-48 h according to various steps. For O2 evolution the initial step was independent of protein synthesis, but the later steps needed de novo synthesis. Concerning recovery of PBS to PS II energy transfer, light seems to be necessary, but neither PS II functioning nor de novo protein synthesis were required. Similarly, light, rather than functional PS II, was important for the recovery of an efficient energy transfer in nitrate-starved cells upon readdition of nitrate. In addition, it has been shown that normal phycobilisomes could accumulate in a Synechocystis sp. PCC 6803 mutant deficient in Photosystem II activity.

Accumulation of chloroplast psbB RNA requires a nuclear factor in Chlamydomonas reinhardtii

We have isolated and characterized a nuclear mutant, 222E, in Chlamydomonas reinhardtii, which is defective in photosystem II (PSII). Polypeptide P5, the product of psbB, is not produced in this mutant, leading to a destabilization of other PSII components. The mutant specifically fails to accumulate psbB transcripts and displays an altered transcription pattern downstream of psbB. Pulse-labelling experiments suggest that mRNA stability and/or processing are affected by the alteration of a nuclear gene product in this mutant. We show that the C. reinhardtii psbB gene is co-transcribed with a small open reading frame that is highly conserved in location and amino acid sequence in land plants. The 5' and 3' termini of the psbB transcript have been mapped to 35 bases upstream of the initiation codon and approximately 600 bases downstream of the stop codon. The 3' flanking region contains two potential stem-loops, of which the larger (with an estimated free energy of -46 kcal) is near the 3' terminus of the transcript.

Oligonucleotide-directed mutagenesis of psbB, the gene encoding CP47, employing a deletion mutant strain of the cyanobacterium Synechocystis sp. PCC 6803

A mutant strain of the cyanobacterium Synechocystis sp. PCC (Pasteur Culture Collection) 6803 has been developed in which psbB, the gene coding for the chlorophyl alpha-binding protein CP47 in Photosystem II (PSII), has been deleted. This deletion mutant can be used for the reintroduction of modified psbB into the cyanobacterium. To study the role of a large hydrophilic region in CP47, presumably located on the lumenal side of the thylakoid membrane between the fifth and sixth membrane-spanning regions, specific deletions have been introduced in psbB coding for regions within this domain. One psbB mutation leads to deletion of Gly-351 to Thr-365 in CP47, another psbB mutation was targeted towards deletion of Arg-384 to Val-392 in this protein. The deletion from Gly-351 to Thr-365 results in a loss of PSII activity and of photoautotrophic growth of the mutant, but the deletion between Arg-384 and Val-392 retains PSII activity and the ability to grow photoautotrophically. The mutant strain with the deletion from Gly-351 to Thr-365 does not assemble a stable PSII reaction center complex in its thylakoid membranes, and exhibits diminished levels of CP47 and of the reaction center proteins D1 and D2. In contrast to the Arg-384 to Val-392 portion of this domain, the region between Gly-351 and Thr-365 appears essential for the normal structure and function of photosystem II.

Evidence that carotenoids are required for the accumulation of a functional photosystem II, but not photosystem I in the cotyledons of mustard seedlings

If Norflurazon-treated mustard (Sinapis alba L.) seedlings are grown in low-fluence-rate white light, accumulation of carotenoids is completely inhibited, while levels of chlorophyll (Chl) a and b are comparable to those of control seedlings. Measurements of fluorescence yield and oxygen evolution indicate that carotenoid-free, green cotyledons are unable to perform leephotosynthesis in vivo. When thylakoid membranes were prepared and electron transport was measured in vitro, only PSI but not PSII activity was detected. Solubilization of the photosystems from thylakoid membranes and separation by sucrose-gradient centrifugation confirmed that PSII is absent in carotenoid-free seedlings, while PSI is present. Western blot analysis for representative proteins of the four photosynthetic complexes showed that subunits 1 and 2 of PSI, the Rieske-iron sulfur-protein, the α-subunit of the CF1 moiety of the ATP-synthase complex, cytochrome b 559 and the lumenal 33-kDa protein of the water-splitting apparatus of PSII are present in comparable amounts in Norflurazon-treated and control plants, while the amounts of Chl-binding proteins of PSII (the major light-harvesting Chl-a/b-binding protein of the antenna complex and the 51- and 44-kDa Chl-a-binding proteins) and two components of the PSII reaction center, (the D1 and D2 protein) are substantially reduced. The data indicate that accumulation of PSII polypeptides is either not inhibited or not completely inhibited in carotenoid-free mustard seedlings, but that assembly of a functional PSII complex does not occur. If Norflurazon-treated seedlings are transferred to water, lutein accumulates rapidly and reaches about 80% of the level detectable in control plants, while the level of other carotenoids is still less than 1%. The accumulation kinetics for lutein are similar to the kinetics for the appearance of PSII activity. This indicates that the availibility of lutein rather than that of other carotenoids might be rate-limiting for the appearance of PSII activity.

Light-activated heterotrophic growth of the cyanobacterium Synechocystis sp. strain PCC 6803: a blue-light-requiring process

A glucose-tolerant strain of Synechocystis sp. strain 6803 will not grow on glucose under complete darkness unless given a daily pulse of white light, typically 5 min of 40 mumol m-2 s-1 (light-pulsed conditions). The light pulse is insufficient for photoautotrophy, as glucose is required and growth yield is dependent on glucose concentration. Growth rate is independent of fluence, but growth yield is dependent on fluence, saturating at 40 to 75 mumol m-2 s-1. A Synechocystis strain 6803 psbA mutant strain grows under light-pulsed conditions at rates similar to those for the glucose-tolerant strain, indicating that photosystem II is not required for growth. The relative spectral sensitivity of the growth of light-pulsed cultures (growth only in blue light, 400 to 500 nm, maximum at 450 nm) precludes energetic contribution from cyclic electron transport around photosystem I. Pulses of long-wavelength light (i.e., 550 and 650 nm) did not support the growth of Synechocystis strain 6803 and, when supplied before or after a blue-light pulse, did not inhibit blue-light-stimulated growth of Synechocystis strain 6803. We conclude that the required blue-light pulse does not support growth via photosynthetic electron transport but appears instead to function as an environmental signal regulating heterotrophic metabolism, cell division, or other photomorphogenic processes. We have termed the growth of Synechocystis strain 6803 pulsed with light and kept otherwise in complete darkness light-activated heterotrophic growth. This observation of a blue-light requirement for the growth of Synechocystis strain 6803 represents a novel blue light effect on the growth of a cyanobacterium.

The structure and function of CPa-1 and CPa-2 in Photosystem II

This review presents a summary of recent investigations examining the structure and function of the chlorophyll-proteins CPa-1 (CP47) and CPa-2 (CP43). Comparisons of the derived amino acid sequences of these proteins suggest sites for chlorophyll binding and for interactions between these chlorophyll-proteins and other Photosystem II components. Hydropathy plot analysis of these proteins allows the formulation fo testable hypotheses concerning their topology and orientation within the photosynthetic membrane. The role of these chlorophyll-proteins as interior light-harvesting chlorophyll-a antennae for Photosystem II is examined and other possible additional roles for these important Photosystem II components are discussed.

The psbC start codon in Synechocystis sp. PCC 6803

The translation start codon for psbC, the gene encoding CP43, a chlorophyll-binding protein of photosystem II, has been identified for the cyanobacterium Synechosystis sp. PCC 6803 using site-directed mutagenesis. An AUG codon, about 50 bases upstream from the end of psbD-I had previously been assumed to be the translation start site of psbC. However, the fact that the AUG codon is not present in psbC from several other organisms, whereas a GUG codon 14 bases upstream from the end of psbD-I is strictly conserved suggests that CP43 translation starts at the latter codon. Mutation of GUG, but not of AUG, led to a loss of CP43 and photoautotrophic growth, indicating that the GUG codon is the sole initiation site for translation of the CP43 protein in Synechocystis sp. PCC 6803.

A cyanobacterial mutant requiring the expression of ribulose bisphosphate carboxylase from a photosynthetic anaerobe

Ribulose bisphosphate carboxylase is essential for both photoautotrophic and photoheterotrophic growth of the cyanobacterium Synechocystis 6803. However, a mutant lacking cyanobacterial carboxylase could be obtained by replacing the natural carboxylase gene with the corresponding gene from Rhodospirillum rubrum, a photosynthetic anaerobe. This treatment produced an organism whose growth depended on the activity of the structurally and functionally dissimilar foreign carboxylase. As a further consequence of this mutagenic replacement, the mutant also lacked microscopically observable carboxysomes, the subcellular inclusion bodies in which the wild-type carboxylase naturally resides. The mutant, dependent on a carboxylase with an inferior relative specificity for CO2 versus O2 and apparently lacking carboxysomes, is extremely sensitive to the CO2/O2 ratio supplied during growth and is unable to grow at all in air. This response to the gas composition should prove useful for selection of various R. rubrum carboxylase mutants with altered specificities for CO2 and O2.

Spectral Properties and Composition of Reaction Center and Ancillary Polypeptide Complexes of Photosystem II Deficient Mutants of Synechocystis 6803

The polypeptide composition and spectral properties of three photosystem II (PSII) deficient mutants of the cyanobacterium Synechocystis 6803 have been determined. The levels of the 43 and 47 kilodalton chlorophyll-binding proteins and the reaction center component D2 are affected differently in each mutant; the 33 kD polypeptide of the oxygen-evolving complex is found at wild-type levels in all three. The 43 and 47 kilodalton proteins are implicated as important elements in the assembly and/or stability of the PSII reaction center, although the loss of one of these polypeptides does not lead to the loss of all PSII proteins. Low temperature fluorescence emission spectra of wild-type cells reveal chlorophyll-attributable peaks at 687 (PSII), 696 (PSII), and 725 (photosystem I) nanometers. All three mutants retain the 725 nanometer fluorescence but lack the 696 nanometer peak. This suggests that the latter fluorescence arises from PSII reaction center chlorophyll or results from interactions among functional PSII components in vivo. Cells that contain the 43 kilodalton and lack the 47 kilodalton protein, retain the 687 fluorescence; furthermore, in as much as this fluorescence is absent from cells without the 43 kilodalton protein, the 687 nanometer peak is judged to emanate from the 43 kilodalton chlorophyll-protein. A new peak, probably previously obscured, is revealed at 691 nanometers in cells that retain the 47 kilodalton protein but lack the 43 kilodalton polypeptide, suggesting that emission near 691 nanometers can be attributed to the 47 kilodalton polypeptide. Membrane-bound phycobilisomes are retained in these cells as is coupled-energy transfer between phycocyanin and allophycocyanin. Energy transfer to photosystem I by way of phycocyanin excitation proceeds as in wild-type cells despite the absence of certain PSII components.

Insertional mutagenesis by random cloning of antibiotic resistance genes into the genome of the cyanobacterium Synechocystis strain PCC 6803

The facultative heterotrophic cyanobacterium Synechocystis sp. strain PCC 6803 was transformed by HaeII Cmr fragments ligated at random to HaeII DNA fragments of the host genome. A similar transformation was done with an AvaII Kmr marker ligated to AvaII host DNA fragments. Integration of the resistance markers into the host genome led to a high frequency of stable Kmr and Cmr transformants. Physical analysis of individual transformants indicated that this result was due to homologous recombination by conversionlike events leading to insertion of the Cmr (or Kmr) gene between two HaeII (or AvaII) sites of the host genome, with precise deletion of the host DNA between these sites. In contrast, integrative crossover of circular DNA molecules with homology to the host DNA is very rare in this cyanobacterium. Strain PCC 6803 was shown to have about 12 genomic copies per cell in standard growth conditions, which complicates the detection of recessive mutations induced by chemical or UV mutagenesis. Random disruption of the host DNA by insertional transformation provides a convenient alternative to transposon mutagenesis in cyanobacteria and may help to overcome the difficulties encountered in generating recessive mutants by classical mutagenesis.

Deletion Mutagenesis of the Cytochrome b559 Protein Inactivates the Reaction Center of Photosystem II

In green plant-like photosynthesis, oxygen evolution is catalyzed by a thylakoid membrane-bound protein complex, photosystem II. Cytochrome b559, a protein component of the reaction center of this complex, is absent in a genetically engineered mutant of the cyanobacterium, Synechocystis 6803 [Pakrasi, H.B., Williams, J.G.K., and Arntzen, C.J. (1988). EMBO J. 7, 325-332]. In this mutant, the genes psbE and psbF, encoding cytochrome b559, were deleted by targeted mutagenesis. Two other protein components, D1 and D2 of the photosystem II reaction center, are also absent in this mutant. However, two chlorophyll-binding proteins, CP47 and CP43, as well as a manganese-stabilizing extrinsic protein component of photosystem II are stably assembled in the thylakoids of this mutant. Thus, this deletion mutation destabilizes the reaction center of photosystem II only. The mutant also lacks a fluorescence maximum peak at 695 nm (at 77 K) even though the CP47 protein, considered to be the origin of this fluorescence peak, is present in this mutant. We propose that the fluorescence at 695 nm originates from an interaction between the reaction center of photosystem II and CP47. The deletion mutant shows the absence of variable fluorescence at room temperature, indicating that its photosystem II complex is photochemically inactive. Also, photoreduction of QA, the primary acceptor quinone in photosystem II, could not be detected in the mutant. We conclude that cytochrome b559 plays at least an essential structural role in the reaction center of photosystem II.

Deletion Mutagenesis of the Cytochrome b559 Protein Inactivates the Reaction Center of Photosystem II

In green plant-like photosynthesis, oxygen evolution is catalyzed by a thylakoid membrane-bound protein complex, photosystem II. Cytochrome b559, a protein component of the reaction center of this complex, is absent in a genetically engineered mutant of the cyanobacterium, Synechocystis 6803 [Pakrasi, H.B., Williams, J.G.K., and Arntzen, C.J. (1988). EMBO J. 7, 325-332]. In this mutant, the genes psbE and psbF, encoding cytochrome b559, were deleted by targeted mutagenesis. Two other protein components, D1 and D2 of the photosystem II reaction center, are also absent in this mutant. However, two chlorophyll-binding proteins, CP47 and CP43, as well as a manganese-stabilizing extrinsic protein component of photosystem II are stably assembled in the thylakoids of this mutant. Thus, this deletion mutation destabilizes the reaction center of photosystem II only. The mutant also lacks a fluorescence maximum peak at 695 nm (at 77 K) even though the CP47 protein, considered to be the origin of this fluorescence peak, is present in this mutant. We propose that the fluorescence at 695 nm originates from an interaction between the reaction center of photosystem II and CP47. The deletion mutant shows the absence of variable fluorescence at room temperature, indicating that its photosystem II complex is photochemically inactive. Also, photoreduction of QA, the primary acceptor quinone in photosystem II, could not be detected in the mutant. We conclude that cytochrome b559 plays at least an essential structural role in the reaction center of photosystem II.

Mutagenesis by random cloning of an Escherichia coli kanamycin resistance gene into the genome of the cyanobacterium Synechocystis PCC 6803: selection of mutants defective in photosynthesis

Photosynthetic mutants of the cyanobacterium Synechocystis PCC 6803 were produced by a random cartridge mutagenesis method leading to gene inactivation. This procedure relies on random ligation of an Escherichia coli kanamycin resistance (Kmr) gene to restriction fragments of genomic DNA from the host. Then recombination occurring during transformation promotes integration of the marker gene into the genome of the recipient cells. Several mutants impaired in photosynthesis were obtained by this procedure. All are partially or totally defective in photosystem II activity and some of them also harbour a functionally modified photosystem I. Restriction and recombination data showed that one mutant (AK1) is best explained as an insertion of the Kmr gene into an AvaII restriction site of the gene psbD-1. All others harbour a deletion, ranging from at least 1.15 kb (AK3) to more than 50 kb (AK9), which partly or fully overlaps the genes psbB and/or psbD-1, depending on the mutant. A genetic-physical map of the more than 60 kb region of the cyanobacterial genome harbouring the genes psbB, psbC and psbD-1 was constructed by combining published sequence data on these genes with the results of recombination and restriction mapping.

The chloroplast thylakoid membrane system is a molecular conveyor belt

Light drives photosynthesis, but paradoxically light is also the most variable environmental factor influencing photosynthesis both qualitatively and quantitatively. The photosynthetic apparatus of higher plants is adaptable in the extreme, as exemplified by its capacity for acclimation to very bright sunny or deeply shaded conditions. It can also respond to rapid changes in light such as sunflecks. In this paper I offer a model that i) explains the thylakoid membrane organisation into grana stacks and stroma lamellae, ii) proposes a role for rapid D1 protein turnover and LHCII phosphorylation, and iii) suggests a mechanism for photoinhibition. I argue that the photosynthetic membrane system is dynamic in three dimensions, so much so that, in the light, it is in constant motion and operates in a manner somewhat analogous to a conveyor belt. D1 protein degradation is proposed to be the motor that drives this system. Photoinhibition is suggested to be due to the arrest of D1 protein turnover.

Bicarbonate effect on electron flow in a cyanobacteriumSynechocystis PCC 6803

In this communication, evidence is presented from the kinetics of QA (-) decay (where QA is the first plastoquinone electron acceptor of photosystem II) and oxygen evolution for the requirement of bicarbonate in the electron transport in a cyanobacteriumSynechocystis (Pasteur Culture Collection 6803). A large slowing down of QA (-) oxidation, measured from the variable chlorophylla fluorescence after saturating actinic flashes, was observed in the thylakoids ofSynechocystis 6803 depleted of bicarbonate in the presence of 25 mM formate. Qualitatively similar results were obtained with DCMU-treated thylakoids. This shows that bicarbonate depletion inhibits electron transport on the acceptor side of photosystem II between QA and the plastoquinone (PQ) pool in cyanobacteria. Addition of 2.5 mM HCO3 (-) fully reversed the inhibition of electron flow caused by bicarbonate depletion. Two exponential phases of QA (-) decay, a fast one and a slow one, were observed with halftimes of approx. 400 μs (fast) and 26 ms (slow) at pH 6.5. At pH 7.5, these phases were approx. 330 μs (fast) and 21 ms (slow), respectively. The amplitude, but not the halftime, of the fast component decreased by about 70% (pH 6.5) or 50% (pH 7.5); this was accompanied by a concomittant increase in the slow phase. Twenty mM bicarbonate stimulated, by a factor of 4, the Hill reaction in bicarbonate-depletedSynechocystis cells. This effect is independent of CO2 fixation as it was observed even in the presence of an inhibitor DBMIB.

Formation of the chlorophyll precursor delta-aminolevulinic acid in cyanobacteria requires aminoacylation of a tRNAGlu species

In the chloroplasts of higher plants and algae, the biosynthesis of the chlorophyll precursor delta-aminolevulinic acid (ALA) involves at least three enzymes and a tRNA species. Here we demonstrate that in cell extracts of the unicellular cyanobacterium Synechocystis sp. strain PCC 6803 ALA was formed from glutamate in a series of reactions in which activation of glutamate by glutamyl-tRNAGlu formation was the first step. The activated glutamate was reduced by a dehydrogenase which displayed tRNA sequence specificity. Fractionation of strain 6803 tRNA by reverse-phase chromatography and polyacrylamide gel electrophoresis yielded two pure tRNAGlu species which stimulated ALA synthesis in vitro. These tRNAs had identical primary sequences but differed in the nucleotide modification of their anticodon. The 6803 tRNAGlu was similar to the sequences of tRNAGlu species or tRNAGlu genes from Escherichia coli and from chloroplasts of Euglena gracilis and higher plants. Southern blot analysis revealed at least two tRNAGlu gene copies in the 6803 chromosome. A glutamate-1-semialdehyde aminotransferase, the terminal enzyme in the conversion of glutamate to ALA in chloroplasts, was detected in 6803 cell extracts by the conversion of glutamate-1-semialdehyde to ALA and by the inhibition of this reaction by gabaculin.

Protein composition of the photosystem II core complex in genetically engineered mutants of the cyanobacterium Synechocystis sp. PCC 6803

The presence of four photosystem II proteins, CP47, CP43, D1 and D2, was monitored in mutants of Synechocystis sp. PCC 6803 that have modified or inactivated genes for CP47, CP43, or D2. It was observed that: (1) thylakoids from mutants without a functional gene encoding CP47 are also depleted in D1 and D2; (2) inactivation of the gene for CP43 leads to decreased but significant levels of CP47, D1 and D2; (3) deletion of part of both genes encoding D2, together with deletion of part of the CP43-encoding gene causes a complete loss of CP47 and D1; (4) thylakoids from a site-directed mutant in which the His-214 residue of D2 has been replaced by asparagine do not contain detectable photosystem II core proteins. However, in another site-directed mutant, in which His-197 has been replaced by tyrosine, some CP47 as well as breakdown products of CP43, but no D1 and D2, can be detected. These data could indicate a central function of CP47 and D2 in stable assembly of the photosystem II complex. CP43, however, is somewhat less critical for formation of the core complex, although CP43 is required for a physiologically functional photosystem II unit. A possible model for the assembly of the photosystem II core complex is proposed.

Protein composition of the photosystem II core complex in genetically engineered mutants of the cyanobacterium Synechocystis sp. PCC 6803

The presence of four photosystem II proteins, CP47, CP43, D1 and D2, was monitored in mutants of Synechocystis sp. PCC 6803 that have modified or inactivated genes for CP47, CP43, or D2. It was observed that: (1) thylakoids from mutants without a functional gene encoding CP47 are also depleted in D1 and D2; (2) inactivation of the gene for CP43 leads to decreased but significant levels of CP47, D1 and D2; (3) deletion of part of both genes encoding D2, together with deletion of part of the CP43-encoding gene causes a complete loss of CP47 and D1; (4) thylakoids from a site-directed mutant in which the His-214 residue of D2 has been replaced by asparagine do not contain detectable photosystem II core proteins. However, in another site-directed mutant, in which His-197 has been replaced by tyrosine, some CP47 as well as breakdown products of CP43, but no D1 and D2, can be detected. These data could indicate a central function of CP47 and D2 in stable assembly of the photosystem II complex. CP43, however, is somewhat less critical for formation of the core complex, although CP43 is required for a physiologically functional photosystem II unit. A possible model for the assembly of the photosystem II core complex is proposed.