Redox control of psbA expression in cyanobacteria Synechocystis strains

Cyanobacterial phytochrome2 regulates the heterotrophic metabolism and has a function in the heat and high-light stress response

Cyanobacteria combine the photosynthetic and respiratory electron transport in one membrane system, the thylakoid membrane. This feature requires an elaborate regulation mechanism to maintain a certain redox status of the electron transport chain, hence allowing proper photosynthetic and respiratory energy metabolism. In this context, metabolic adaptations, as seen in the light-to-dark and dark-to-light transitions, are particularly challenging. However, the molecular basis of the underlying regulatory mechanisms is not well-understood. Here, we describe a function of cyanobacterial phytochrome2 (Cph2), a phytochrome of the cyanobacterial model system Synechocystis sp. PCC 6803, in regulation of the primary energy metabolism. When cells are shifted from photoautotrophic planktonic growth to light-activated heterotrophic growth and biofilm initiation, knockout of Cph2 results in impaired growth, a decrease in the activity of Glc-6-P dehydrogenase, a decrease of the transcript abundance/activity of cytochrome-c-oxidase, and slower phycocyanin degradation. Measurements of the plastoquinone reduction confirm an impaired heterotrophic metabolism in the cph2 knockout. When cells that were adapted to heterotrophic metabolism are shifted back to light conditions, the knockout of Cph2 results in an altered photosystem II chlorophyll fluorescence induction curve, which is indicative of an impaired redox balance of the electron transport chain. Moreover, Cph2 plays a role in the heat and high-light stress response, particularly under photomixotrophic conditions. Our results show a function of Cph2 in the adaptation of the primary energy metabolism to changing trophic conditions. The physiological role of Cph2 in biofilm formation is discussed.

Transcript patterns of chloroplast-encoded genes in cultured Symbiodinium spp. (Dinophyceae): testing the influence of a light shift and diel periodicity

Microalgae possess numerous cellular mechanisms specifically employed for acclimating the photosynthetic pathways to changes in the physical environment. Despite the importance of coral-dinoflagellate symbioses, little focus has been given as to how the symbiotic algae (Symbiodinium spp.) regulate the expression of their photosynthetic genes. This study used real-time PCR to investigate the transcript abundance of the plastid-encoded genes, psbA (encoding the D1 protein of photosystem II) and psaA (encoding the P700 protein in photosystem I), within the cultured Symbiodinium ITS-2 (internal transcribed spacer region) types A20 and A13. Transcript abundance was monitored during a low to high-light shift, as well as over a full diel light cycle. In addition, psaA was characterized in three isolates (A20, A13, and D4-5) and noted as another example of a dinoflagellate plastid gene encoded on a minicircle. In general, the overall incongruence of transcript patterns for both psbA and psaA between the Symbiodinium isolates and other models of transcriptionally controlled chloroplast gene expression (e.g., Pisum sativum [pea], Sinapis alba [mustard seedling], and Synechocystis sp. PCC 6803 [cyanobacteria]) suggests that Symbiodinium is reliant on posttranscriptional mechanisms for homeostatic regulation of its photosynthetic proteins.

Acclimation to high-light conditions in cyanobacteria: from gene expression to physiological responses

Photosynthetic organisms have evolved various acclimatory responses to high-light (HL) conditions to maintain a balance between energy supply (light harvesting and electron transport) and consumption (cellular metabolism) and to protect the photosynthetic apparatus from photodamage. The molecular mechanism of HL acclimation has been extensively studied in the unicellular cyanobacterium Synechocystis sp. PCC 6803. Whole genome DNA microarray analyses have revealed that the change in gene expression profile under HL is closely correlated with subsequent acclimatory responses such as (1) acceleration in the rate of photosystem II turnover, (2) downregulation of light harvesting capacity, (3) development of a protection mechanism for the photosystems against excess light energy, (4) upregulation of general protection mechanism components, and (5) regulation of carbon and nitrogen assimilation. In this review article, we survey recent progress in the understanding of the molecular mechanisms of these acclimatory responses in Synechocystis sp. PCC 6803. We also briefly describe attempts to understand HL acclimation in various cyanobacterial species in their natural environments.

Strategies for psbA gene expression in cyanobacteria, green algae and higher plants: from transcription to PSII repair

The Photosystem (PS) II of cyanobacteria, green algae and higher plants is prone to light-induced inactivation, the D1 protein being the primary target of such damage. As a consequence, the D1 protein, encoded by the psbA gene, is degraded and re-synthesized in a multistep process called PSII repair cycle. In cyanobacteria, a small gene family codes for the various, functionally distinct D1 isoforms. In these organisms, the regulation of the psbA gene expression occurs mainly at the level of transcription, but the expression is fine-tuned by regulation of translation elongation. In plants and green algae, the D1 protein is encoded by a single psbA gene located in the chloroplast genome. In chloroplasts of Chlamydomonas reinhardtii the psbA gene expression is strongly regulated by mRNA processing, and particularly at the level of translation initiation. In chloroplasts of higher plants, translation elongation is the prevalent mechanism for regulation of the psbA gene expression. The pre-existing pool of psbA transcripts forms translation initiation complexes in plant chloroplasts even in darkness, while the D1 synthesis can be completed only in the light. Replacement of damaged D1 protein requires also the assistance by a number of auxiliary proteins, which are encoded by the nuclear genome in green algae and higher plants. Nevertheless, many of these chaperones are conserved between prokaryotes and eukaryotes. Here, we describe the specific features and fundamental differences of the psbA gene expression and the regeneration of the PSII reaction center protein D1 in cyanobacteria, green algae and higher plants. This article is part of a Special Issue entitled Photosystem II.

The role of carotenoid isomerase in maintenance of photosynthetic oxygen evolution in rice plant

Carotenoid isomerase (CRTISO) has been suggested to protect photosystem II (PS II) from photodamage, probably through its product lutein. However, the mechanism of the photoprotection still remains to be further elucidated. In this work, we cloned a point mutated gene reported to encode a CRTISO which is responsible for the accumulation of lutein in rice mutant zel1 by a map-based cloning approach. The mutant phenotype was rescued by transformation with the corresponding gene of the wild type (WT). The activity of photosynthetic oxygen evolution was evidently suppressed in zel1. The amount of the core protein of PS II CP47 was much lower in all the PS II complexes especially in the LHCII-PS II supercomplexes and CP43-free PS II of zel1 than that of WT. On the other hand, the amount of another core protein of PS II CP43 of zel1 was decreased in the higher supercomplexes, whereas it was increased in the lower ones and PS II monomer. The immunodetection displayed that CP43, CP47, and the oxygen-evolving extrinsic proteins PsbO and PsbP were reduced, but the amount of reaction center protein D1 did not show significant change in zel1. Northern blot analysis showed that the transcriptional level of CP43 was down-regulated but not that of CP47 or D1 in zel1. In addition, the plastoquinone (PQ) Q(A) was in a reduced state in zel1. On the basis of the results, we suggest that CRTISO might function in regulating the transcription of CP43 and the translation of CP47 by affecting the redox state of the PQ to stabilize the extrinsic proteins of oxygen evolution complexes in the rice plant.

Cyanobacterial psbA gene family: optimization of oxygenic photosynthesis

The D1 protein of Photosystem II (PSII), encoded by the psbA genes, is an indispensable component of oxygenic photosynthesis. Due to strongly oxidative chemistry of PSII water splitting, the D1 protein is prone to constant photodamage requiring its replacement, whereas most of the other PSII subunits remain ordinarily undamaged. In cyanobacteria, the D1 protein is encoded by a psbA gene family, whose members are differentially expressed according to environmental cues. Here, the regulation of the psbA gene expression is first discussed with emphasis on the model organisms Synechococcus sp. and Synechocystis sp. Then, a general classification of cyanobacterial D1 isoforms in various cyanobacterial species into D1m, D1:1, D1:2, and D1' forms depending on their expression pattern under acclimated growth conditions and upon stress is discussed, taking into consideration the phototolerance of different D1 forms and the expression conditions of respective members of the psbA gene family.

The long-term response to fluctuating light quality is an important and distinct light acclimation mechanism that supports survival of Arabidopsis thaliana under low light conditions

The long-term response (LTR) of higher plants to varying light qualities increases the photosynthetic yield; however, the benefit of this improvement for physiology and survival of plants is largely unknown, and its functional relation to other light acclimation responses has never been investigated. To unravel positive effects of the LTR we acclimated Arabidopsis thaliana for several days to light sources, which preferentially excite photosystem I (PSI) or photosystem II (PSII). After acclimation, plants revealed characteristic differences in chlorophyll fluorescence, thylakoid membrane stacking, phosphorylation state of PSII subunits and photosynthetic yield of PSII and PSI. These LTR-induced changes in the structure, function and efficiency of the photosynthetic machinery are true effects by light quality acclimation, which could not be induced by light intensity variations in the low light range. In addition, high light stress experiments indicated that the LTR is not involved in photoinhibition; however, it lowers non-photochemical quenching (NPQ) by directing more absorbed light energy into photochemical work. NPQ in turn is not essential for the LTR, since npq mutants performed a normal acclimation. We quantified the beneficial potential of the LTR by comparing wild-type plants with the LTR-deficient mutant stn7. The mutant exhibited a decreased effective quantum yield and produced only half of seeds when grown under fluctuating light quality conditions. Thus, the LTR represents a distinct acclimation response in addition to other already known responses that clearly improves plant physiology under low light conditions resulting in a pronounced positive effect on plant fitness.

Redox of plastoquinone pool regulates the expression and activity of NADPH dehydrogenase supercomplex in Synechocystis sp. strain PCC 6803

A highly active NADPH dehydrogenase supercomplex, which is essential for cyclic electron transport around photosystem I (cyclic PSI) and respiration, was newly identified in cyanobacteria. Synechocystis sp. strain PCC 6803 cells were treated with exogenous glucose (Glc) or 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU); subsequently, active staining of NADPH-nitroblue tetrazolium oxidoreductase, western blot, and the initial rate of P700+ dark reduction were assessed in the cyanobacterium at several time points. The expression and enzyme activity levels of NADPH dehydrogenase supercomplex were gradually inhibited and closely associated with the decrease in the rate of cyclic PSI accompanying the addition of exogenous Glc to the cultures. In contrast, the activity levels were significantly stimulated but did not cause an increase in the rate of cyclic PSI as expected in the presence of DCMU. Since Glc results in the partial reduction of the plastoquinone (PQ) pool while DCMU results in the overoxidation of the PQ pool, the present results demonstrate that the expression and activity of NADPH dehydrogenase supercomplex are under the influence of the redox control of the PQ pool while the operation of cyclic PSI as mediated by this supercomplex requires an appropriate redox poise of the PQ pool.

Systematic analysis of the relation of electron transport and ATP synthesis to the photodamage and repair of photosystem II in Synechocystis

The photosynthetic machinery and, in particular, the photosystem II (PSII) complex are susceptible to strong light, and the effects of strong light are referred to as photodamage or photoinhibition. In living organisms, photodamaged PSII is rapidly repaired and, as a result, the extent of photoinhibition represents a balance between rates of photodamage and the repair of PSII. In this study, we examined the roles of electron transport and ATP synthesis in these two processes by monitoring them separately and systematically in the cyanobacterium Synechocystis sp. PCC 6803. We found that the rate of photodamage, which was proportional to light intensity, was unaffected by inhibition of the electron transport in PSII, by acceleration of electron transport in PSI, and by inhibition of ATP synthesis. By contrast, the rate of repair was reduced upon inhibition of the synthesis of ATP either via PSI or PSII. Northern blotting and radiolabeling analysis with [(35)S]Met revealed that synthesis of the D1 protein was enhanced by the synthesis of ATP. Our observations suggest that ATP synthesis might regulate the repair of PSII, in particular, at the level of translation of the psbA genes for the precursor to the D1 protein, whereas neither electron transport nor the synthesis of ATP affects the extent of photodamage.

Antagonistic dark/light-induced SigB/SigD, group 2 sigma factors, expression through redox potential and their roles in cyanobacteria

The expression of group 2 sigma factors is characterized in a cyanobacterium Synechocystis sp. PCC 6803 grown in culture, changing light conditions (white, red and blue light, and darkness), or the presence of drugs (rifampicin, chloramphenicol, DCMU, and DBMIB), and the roles of these sigma factors are elucidated. The expression of dark/light-induced SigB/SigD was accelerated under opposite redox (oxidation/reduction) states in an electron transport chain of photosynthesis. Expression of the dark-induced lrtA and light-induced psbA2/3 transcript was significantly reduced in the sigB and sigD knockout strains, respectively. Abundant amounts of sigB transcript and protein were observed in the sigC knockout strain, implying that SigC represses SigB expression under light. These findings clearly showed that SigB/SigD with another group 2 sigma, SigC, contribute to transcription for a subset of dark/light-responsive genes in the cyanobacterium. A possible model for SigB/SigD is presented and the potential ability for promoter recognition is also discussed.

Role of the PSII-H subunit in photoprotection: novel aspects of D1 turnover in Synechocystis 6803

Photosystem I-less Synechocystis 6803 mutants carrying modified PsbH proteins, derived from different combinations of wild-type cyanobacterial and maize genes, were constructed. The mutants were analyzed in order to determine the relative importance of the intra- and extramembrane domains of the PsbH subunit in the functioning of photosystem (PS) II, by a combination of biochemical, biophysical, and physiological approaches. The results confirmed and extended previously published data showing that, besides D1, the whole PsbH protein is necessary to determine the correct structure of a QB/herbicide-binding site. The different turnover of the D1 protein and chlorophyll photobleaching displayed by mutant cells in response to photoinhibitory treatment revealed for the first time the actual role of the PsbH subunit in photoprotection. A functional PsbH protein is necessary for (i) rapid degradation of photodamaged D1 molecules, which is essential to avoid further oxidative damage to the PSII core, and (ii) insertion of newly synthesized D1 molecules into the thylakoid membrane. PsbH is thus required for both initiation and completion of the repair cycle of the PSII complex in cyanobacteria.

DNA microarray analysis of redox-responsive genes in the genome of the cyanobacterium Synechocystis sp. strain PCC 6803

Whole-genome DNA microarrays were used to evaluate the effect of the redox state of the photosynthetic electron transport chain on gene expression in Synechocystis sp. strain PCC 6803. Two specific inhibitors of electron transport, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), were added to the cultures, and changes in accumulation of transcripts were examined. About 140 genes were highlighted as reproducibly affected by the change in the redox state of the photosynthetic electron transport chain. It was shown that some stress-responsive genes but not photosynthetic genes were under the control of the redox state of the plastoquinone pool in Synechocystis sp. strain PCC 6803.

Two distinct mechanisms regulate the transcription of photosystem II genes in Synechocystis sp. PCC 6803

Expression and regulation of psb genes, encoding various subunits of photosystem II (PSII), were studied in the cyanobacterium Synechocystis sp. PCC 6803. Transcription of the psbA and psbD genes, encoding the PSII reaction centre proteins D1 and D2, was rapidly activated upon onset of illumination and the transcription rates were enhanced at high irradiance. Gel retardation analysis demonstrated dark-enhanced binding of proteins to the upstream region of the psbA2 gene, pointing to a repressor-protein-based transcriptional regulation mechanism. Transcription of all the other psb genes also required light, but unlike the psbA and psbD genes, these psb genes did not respond specifically to high-light. Moreover, the transcription of these psb genes was activated slowly at onset of illumination, and was strictly dependent on de novo protein synthesis. We suggest that these psb genes are up-regulated in the light via transcriptional activator proteins, and the slow activation may be related to production of new PSII centres during growth. Apart from the two distinct mechanisms for transcriptional regulation, all psb genes shared a common regulation mechanism at the level of transcript stability, mediated by the redox poise of intersystem electron carrier(s).

Cold-regulated genes under control of the cold sensor Hik33 in Synechocystis

A histidine kinase, Hik33, appears to sense decreases in temperature and to regulate the expression of certain cold-inducible genes in the cyanobacterium Synechocystis sp. PCC6803. To examine the role of Hik33 in the regulation of gene expression, we analysed a DeltaHik33 mutant using the DNA microarray technique. In wild-type cells, genes that were strongly induced at low temperature encoded proteins that were predominantly subunits of the transcriptional and translational machinery. Most cold-repressible genes encoded components of the photosynthetic machinery. Mutation of the hik33 gene suppressed the expression of some of these cold-regulated genes, which could be divided into three groups according to the effect of the mutation of hik33. In the first group, regulation of gene expression by low temperature was totally abolished; in the second group, the extent of such regulation was reduced by half; and, in the third group, such regulation was totally unaffected. These results suggest that expression of the genes in the first group is regulated solely by Hik33, expression of genes in the third group is regulated by an as yet unidentified cold sensor, and expression of genes in the second group is regulated by both these cold sensors.

Regulation of psbA and psaE expression by light quality in Synechocystis species PCC 6803. A redox control mechanism

We investigated the influence of light of different wavelengths on the expression of the psbA gene, which encodes the D1 protein of the photosystem II and the psaE gene, which encodes the subunit Psa-E of the photosystem I, in Synechocystis sp PCC 6803. In an attempt to differentiate between a light-sensory and a redox-sensory signaling processes, the effect of orange, blue, and far-red light was studied in the wild-type and in a phycobilisome-less mutant. Transferring wild-type cells from one type of illumination to another induced changes in the redox state of the electron transport chain and in psbA and psaE expression. Blue and far-red lights (which are preferentially absorbed by the photosystem I) induced an accumulation of psbA transcripts and a decrease of the psaE mRNA level. In contrast, orange light (which is preferentially absorbed by the photosystem II) induced a large accumulation of psaE transcripts and a decrease of psbA mRNA level. Transferring mutant cells from blue to orange light (or vice versa) had no effect either on the redox state of the electron transport chain or on the levels of psbA and psaE mRNAs. Thus, light quality seems to regulate expression of these genes via a redox sensory mechanism in Synechocystis sp PCC 6803 cells. Our data suggest that the redox state of one of the electron carriers between the plastoquinone pool and the photosystem I has opposite influences on psbA and psaE expression. Its reduction induces accumulation of psaE transcripts, and its oxidation induces accumulation of psbA mRNAs.

Redox control of ntcA gene expression in Synechocystis sp. PCC 6803. Nitrogen availability and electron transport regulate the levels of the NtcA protein

In this work we have studied the influence of the cellular redox status in the expression of the Synechocystis sp. PCC 6803 ntcA gene. Two different ntcA transcripts with different 5' ends were detected, depending on the different dark/light or nitrogen availability conditions. Accumulation of a 0.8-kb ntcA message was light and nitrogen dependent, whereas a longer 1.2-kb ntcA transcript was neither light nor nitrogen regulated. NtcA protein levels increased concomitantly with the accumulation of the 0.8-kb ntcA transcript. The light-dependent accumulation of the ntcA gene and the NtcA protein was sensitive to electron transport inhibitors. In addition, Glc-grown Synechocystis sp. cells showed a similar ntcA expression pattern in darkness to that observed under illumination. These data suggested that electron transport, and not light per se may regulate ntcA gene expression. Primer extension analysis, together with gel mobility-shift assays, demonstrated that in vitro, the Synechocystis sp. NtcA protein specifically bound to the putative promoter region from the light/nitrogen-dependent ntcA transcript but not to that from the constitutive 1.2-kb ntcA mRNA. Band-shift experiments carried out in the presence of thiol oxidizing/modifiying agents and different reducing/oxidizing conditions suggested that NtcA binding to its own promoter was under a thiol-dependent redox mechanism. Our results suggest that the cellular redox status plays a central role in the autoregulatory mechanism of the NtcA protein.

A redox-responsive regulator of photosynthesis gene expression in the cyanobacterium Synechocystis sp. Strain PCC 6803

We have identified genes in the unicellular cyanobacterium Synechocystis sp. strain PCC 6803 that are involved with redox control of photosynthesis and pigment-related genes. The genes, rppA (sll0797) and rppB (sll0798), represent a two-component regulatory system that controls the synthesis of photosystem II (PSII) and PSI genes, in addition to photopigment-related genes. rppA (regulator of photosynthesis- and photopigment-related gene expression) and rppB exhibit strong sequence similarity to prokaryotic response regulators and histidine kinases, respectively. In the wild type, the steady-state mRNA levels of PSII reaction center genes increased when the plastoquinone (PQ) pool was oxidized and decreased when the PQ pool was reduced, whereas transcription of the PSI reaction center genes was affected in an opposite fashion. Such results suggested that the redox poise of the PQ pool is critical for regulation of the photosystem reaction center genes. In Delta rppA, an insertion mutation of rppA, the PSII gene transcripts were highly up-regulated relative to the wild type under all redox conditions, whereas transcription of phycobilisome-related genes and PSI genes was decreased. The higher transcription of the psbA gene in Delta rppA was manifest by higher translation of the D1 protein and a concomitant increase in O(2) evolution. The results demonstrated that RppA is a regulator of photosynthesis- and photopigment-related gene expression, is involved in the establishment of the appropriate stoichiometry between the photosystems, and can sense changes in the PQ redox poise.

Recovery of photosystem II activity in photoinhibited synechocystis cells: light-dependent translation activity is required besides light-independent synthesis of the D1 protein

Irreversible photoinactivation of photosystem II (PSII) results in the degradation of the reaction center II D1 protein. In Synechocystis PCC 6714 cells, recovery of PSII activity requires illumination. The rates of photoinactivation and recovery of PSII activity in the light are similar in cells grown in minimal (MM) or glucose-containing medium (GM). Reassembly of PSII with newly synthesized proteins requires degradation of the D1 protein of the photoinactivated PSII. This process may occur in darkness in both types of cells. The degraded D1 protein is, however, only partially replaced by newly synthesized protein in MM cells in darkness while a high level of D1 protein synthesis occurs in darkness in the GM cells. The newly synthesized D1 protein in darkness appears to be assembled with other PSII proteins. However, PSII activity is not recovered in such cells. Illumination of the cells in absence but not in the presence of protein synthesis inhibitors allows recovery of PSII activity.

Redox control of psbA gene expression in the cyanobacterium Synechocystis PCC 6803. Involvement of the cytochrome b(6)/f complex

We investigated the role of the redox state of the photosynthetic and respiratory electron transport chains on the regulation of psbA expression in Synechocystis PCC 6803. Different means to modify the redox state of the electron carriers were used: (a) dark to oxidize the whole electron transport chain; (b) a shift from dark to light to induce its reduction; (c) the chemical interruption of the electron flow at different points to change the redox state of specific electron carriers; and (d) the presence of glucose to maintain a high reducing power in darkness. We show that changes in the redox state of the intersystem electron transport chain induce modifications of psbA transcript production and psbA mRNA stability. Reduction of the intersystem electron carriers activates psbA transcription and destabilizes the mRNA, while their oxidation induces a decrease in transcription and a stabilization of the transcript. Furthermore, our data suggest that the redox state of one of the electron carriers between the plastoquinone pool and photosystem I influences not only the expression of the psbA gene, but also that of other two photosynthetic genes, psaE and cpcBA. As a working hypothesis, we propose that the occupancy of the Q(0) site in the cytochrome b(6)/f complex may be involved in this regulation.