Blue and red light reversibly control psbA expression in the cyanobacterium Synechococcus sp. strain PCC 7942

Blue light induces major changes in the gene expression profile of the cyanobacterium Synechocystis sp. PCC 6803

Although cyanobacteria absorb blue light, they use it less efficiently for photosynthesis than other colors absorbed by their photosynthetic pigments. A plausible explanation for this enigmatic phenomenon is that blue light is not absorbed by phycobilisomes and, hence, causes an excitation shortage at photosystem II (PSII). This hypothesis is supported by recent physiological studies, but a comprehensive understanding of the underlying changes in gene expression is still lacking. In this study, we investigate how a switch from artificial white light to blue, orange or red light affects the transcriptome of the cyanobacterium Synechocystis sp. PCC 6803. In total, 145 genes were significantly regulated in response to blue light, whereas only a few genes responded to orange and red light. In particular, genes encoding the D1 and D2 proteins of PSII, the PSII chlorophyll-binding protein CP47 and genes involved in PSII repair were upregulated in blue light, whereas none of the photosystem I (PSI) genes responded to blue light. These changes were accompanied by a decreasing PSI:PSII ratio. Furthermore, many genes involved in gene transcription and translation and several ATP synthase genes were transiently downregulated, concurrent with a temporarily decreased growth rate in blue light. After 6-7 days, when cell densities had strongly declined, the growth rate recovered and the expression of these growth-related genes returned to initial levels. Hence, blue light induces major changes in the transcriptome of cyanobacteria, in an attempt to increase the photosynthetic activity of PSII and cope with the adverse growth conditions imposed by blue light.

Analysis of the Fine-Tuning of Cyanobacterial Circadian Phase by Monochromatic Light and Long-Day Conditions

The cyanobacterial circadian-related protein, Pex, accumulates in the dark period of the diurnal light-dark cycle. After the diurnal cycle, an approximately 3 h advance in the phase of the circadian bioluminescence rhythm is observed in pex-deficient mutants, as compared with the wild type. However, it is unclear what type of photosensing mechanism regulates the accumulation and the phase change. In monochromatic light irradiation experiments, Pex accumulation was strongly repressed under blue light conditions; however, only small reductions in Pex accumulation were observed under red or green light conditions. After the diurnal cycle of 12 h of white fluorescent light and 12 h of blue light, the phase advance was repressed more than that of the cycle of 12 h red (or green) light. The phase advance also occurred after 16 h light/8 h dark cycles (long-day cycles) but did not occur after 8 h light/16 h dark cycles (short-day cycles). While Pex is a unique winged helix transcription factor harboring secondary structures (α0 and α4 helices), the importance of the structures is not understood. In in vivo experiments with site-directed mutations in the α0 helix, the obtained mutants, in which Pex was missing the hydrophobic side chain at the 28th or 32nd amino acid residue, exhibited no phase delay after the light/dark cycle. In in vitro DNA binding assays, the mutant proteins showed no binding to the promoter region of the clock gene kaiA. From these results, we propose a molecular model which describes the phase delay in cyanobacteria.

Engineered Photosystem II reaction centers optimize photochemistry versus photoprotection at different solar intensities

The D1 protein of Photosystem II (PSII) provides most of the ligating amino acid residues for the Mn4CaO5 water-oxidizing complex (WOC) and half of the reaction center cofactors, and it is present as two isoforms in the cyanobacterium Synechococcus elongatus PCC 7942. These isoforms, D1:1 and D1:2, confer functional advantages for photosynthetic growth at low and high light intensities, respectively. D1:1, D1:2, and seven point mutations in the D1:2 background that are native to D1:1 were expressed in the green alga Chlamydomonas reinhardtii. We used these nine strains to show that those strains that confer a higher yield of PSII charge separation under light-limiting conditions (where charge recombination is significant) have less efficient photochemical turnover, measured in terms of both a lower WOC turnover probability and a longer WOC cycle period. Conversely, these same strains under light saturation (where charge recombination does not compete) confer a correspondingly faster O2 evolution rate and greater protection against photoinhibition. Taken together, the data clearly establish that PSII primary charge separation is a trade-off between photochemical productivity (water oxidation and plastoquinone reduction) and charge recombination (photoprotection). These trade-offs add up to a significant growth advantage for the two natural isoforms. These insights provide fundamental design principles for engineering of PSII reaction centers with optimal photochemical efficiencies for growth at low versus high light intensities.

Comparison of transcriptome under red and blue light culture of Saccharina japonica (Phaeophyceae)

Saccharina japonica is one of the most important economic seaweeds. Several aspects such as photosynthesis in Saccharina lives are affected by blue light, the predominant light spectrum in the habitat. In this study, transcriptome profiling of S. japonica by next generation sequencing technology generated 55,102 qualified transcripts and 40.5 % transcripts were assigned to functional annotation. Expression of a large proportion of genes has been previously reported to be regulated by blue light, taking dark as control. However, by comparison among white, blue and red light, the significantly differentially expressed gene tags (DEGs) accounted for only 6.75 % of the identified sequences. It indicated that light-regulated gene expression in kelps is not a specific blue-light response. Unexpectedly, red light had more extensive effects on the transcriptomic activity than blue light did, since the most (68.4 %) DEGs were red light-regulated and only 17.5 % were specifically regulated by blue light. Some of the DEGs with the highest mRNA levels under blue light are not blue light-upregulated but red light-downregulated. The extensive regulation on gene expression under red light together with the abundant presence of phytochrome-like protein gene tags in S. japonica indicated their significant roles in the lives of brown algae. By highlighting the photosynthetic metabolism, blue light is more efficient than red light in triggering the pigment biosynthesis, light reaction and carbon fixation, revealing a molecular basis for rapid growth of kelps, since most of the time blue light is predominant in their habitat.

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.

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.

A systems-level analysis of the effects of light quality on the metabolism of a cyanobacterium

Photosynthetic organisms experience changes in light quantity and light quality in their natural habitat. In response to changes in light quality, these organisms redistribute excitation energy and adjust photosystem stoichiometry to maximize the utilization of available light energy. However, the response of other cellular processes to changes in light quality is mostly unknown. Here, we report a systematic investigation into the adaptation of cellular processes in Synechocystis species PCC 6803 to light that preferentially excites either photosystem II or photosystem I. We find that preferential excitation of photosystem II and photosystem I induces massive reprogramming of the Synechocystis transcriptome. The rewiring of cellular processes begins as soon as Synechocystis senses the imbalance in the excitation of reaction centers. We find that Synechocystis utilizes the cyclic photosynthetic electron transport chain for ATP generation and a major part of the respiratory pathway to generate reducing equivalents and carbon skeletons during preferential excitation of photosystem I. In contrast, cytochrome c oxidase and photosystem I act as terminal components of the photosynthetic electron transport chain to produce sufficient ATP and limited amounts of NADPH and reduced ferredoxin during preferential excitation of photosystem II. To overcome the shortage of NADPH and reduced ferredoxin, Synechocystis preferentially activates transporters and acquisition pathways to assimilate ammonia, urea, and arginine over nitrate as a nitrogen source. This study provides a systematic analysis of cellular processes in cyanobacteria in response to preferential excitation and shows that the cyanobacterial cell undergoes significant adjustment of cellular processes, many of which were previously unknown.

Differential regulation of psbA and psbD gene expression, and the role of the different D1 protein copies in the cyanobacterium Thermosynechococcus elongatus BP-1

In Thermosynechococcus elongatus BP-1, which is the preferred organism in recent structural studies of PSII, three psbA and two psbD genes code for three D1 and one D2 protein isoforms, respectively. The regulation and function of these genes and protein products is largely unknown. Therefore, we used quantitative RT-PCR to follow changes in the mRNA level of the respective genes, in combination with biophysical measurements to detect changes in the electron transport activity of Photosystem II under exposure to different visible and UV light, and temperature conditions. In cells which are acclimated to 40 micromol m(-2)s(-1) growth light conditions at 40 degrees C the main populations of the psbA and psbD transcripts arise from the psbA1 and psbD1 genes, respectively. When the temperature is raised to 60 degrees C psbA1 becomes the single dominating psbA mRNA species. Upon exposure of the cells to 500 micromol m(-2)s(-1) intensity visible light psbA3 replaces psbA1 as the dominating psbA mRNA species, and psbD2 increases at the expense of psbD1. UV-B radiation also increases the abundance of psbA3, and psbD2 at the expense of psbA1 and psbD1, respectively. From the different extent of total D1 protein loss in the absence and presence of lincomycin it was estimated that the PsbA3 protein isoform replaces PsbA1 in about 65% of PSII centers after 2 h of high light acclimation. Under the conditions of different psbA transcript distributions chlorophyll fluorescence and thermoluminescence measurements were applied to monitor charge recombination characteristics of the S2Q(A)(-) and S2Q(B)(-) states. We obtained faster decay of flash-induced chlorophyll fluorescence in the presence of DCMU, as well as lower peak temperature of the Q and B thermoluminescence bands when PsbA3 replaced PsbA1 as the main D1 protein isoform. The relevance of dynamic changes in the abundance of psbA and psbD transcript levels, as well as D1 protein isoforms in the acclimation of T. elongatus to changing environmental conditions is discussed.

The psbA gene family responds differentially to light and UVB stress in Gloeobacter violaceus PCC 7421, a deeply divergent cyanobacterium

Gloeobacter violaceus PCC 7421 is a slow-growing cyanobacterium which lacks thylakoid membranes, but whose five-membered psbA gene family encodes three isoform variants of the PsbA (D1) reaction center protein of Photosystem II. Under standard culture conditions Gloeobacter exhibits photosystem II electron transport, but several clear modifications in the redox potential of key cofactors bound by the PsbA protein are manifested in the flash-fluorescence characteristics. In other cyanobacteria dynamic expression of multiple psbA genes and turnover of PsbA isoforms is critical to counter excitation stress. We found that each of Gloeobacter's five psbA genes is expressed, with transcript abundances spanning 4.5 orders of magnitude. psbAI (glr2322) and psbAII (glr0779), encoding identical PsbA:2 form proteins, are constitutively expressed and dominate the psbA transcript pool under control conditions. psbAIII (gll3144) was strongly induced under photoinhibitory high irradiance stress, thereby contributing to a large increase in the psbA transcript pool that allowed cells to maintain their PsbA protein pools and then recover from irradiance stress, within one cellular generation. In contrast, under comparable photoinhibition provoked by UVB the cells were unable to maintain their psbA transcript and PsbA protein pools, and showed limited subsequent recovery. psbAIV (glr1706) and psbAV (glr2656), encoding two divergent PsbA isoforms, showed consistent trace expression but were never quantitatively significant contributors to the psbA transcript pool.

Cyanobacterial psbA families in Anabaena and Synechocystis encode trace, constitutive and UVB-induced D1 isoforms

Cyanobacteria cope with UVB induced photoinhibition of Photosystem II by regulating multiple psbA genes to boost the expression of D1 protein (in Synechocystis sp. PCC6803), or to exchange the constitutive D1:1 protein to an alternate D1:2 isoform (in Synechococcus sp. PCC7942). To define more general patterns of cyanobacterial psbA expression, we applied moderately photoinhibitory UVB to Anabaena sp. PCC7120 and tracked the expression of its five psbA genes. psbAI, encoding a D1:1 protein isoform characterized by a Gln130, represented the majority of the psbA transcript pool under control conditions. psbAI transcripts decreased upon UVB treatment but the total psbA transcript pool increased 3.5 fold within 90 min as a result of sharply increased psbAII, psbAIV and psbAIII transcripts encoding an alternate D1:2 protein isoform characterized by Glu130, similar to that of Synechococcus. Upon UVB treatment the relaxation of flash induced chlorophyll fluorescence showed a characteristic acceleration of a decay phase likely associated with the exchange from the D1:1 protein isoform encoded by psbAI to the alternate D1:2 isoform encoded by psbAIV, psbAII and psbAIII. Throughout the UVB treatment the divergent psbA0 made only a trace contribution to the total psbA transcript pool. This suggests a similarity to the divergent psbAI gene from Synechocystis, whose natural expression we demonstrate for the first time at a trace level similar to psbA0 in Anabaena. These trace-expressed psbA genes in two different cyanobacteria raise questions concerning the functions of these divergent genes.

Biochemical and functional characterization of BLUF-type flavin-binding proteins of two species of cyanobacteria

BLUF (a sensor of Blue-Light Using FAD) is a novel putative photoreceptor domain that is found in many bacteria and some eukaryotic algae. As found on genome analysis, certain cyanobacteria have BLUF proteins with a short C-terminal extension. As typical examples, Tll0078 from thermophilic Thermosynechococcus elongatus BP-1 and Slr1694 from mesophilic Synechocystis sp. PCC 6803 were comparatively studied. FAD of both proteins was hardly reduced by exogenous reductants or mediators except methylviologen but showed a typical spectral shift to a longer wavelength upon excitation with blue light. In particular, freshly prepared Tll0078 protein showed slow but reversible aggregation, indicative of light-induced conformational changes in the protein structure. Tll0078 is far more stable as to heat treatment than Slr1694, as judged from flavin fluorescence. The slr1694-disruptant showed phototactic motility away from the light source (negative phototaxis), while the wild type Synechocystis showed positive phototaxis toward the source. Yeast two-hybrid screening with slr1694 showed self-interaction of Slr1694 (PixD) with itself and interaction with a novel PatA-like response regulator, Slr1693 (PixE). These results were discussed in relation to the signaling mechanism of the "short" BLUF proteins in the regulation of cyanobacterial phototaxis.

Resilient circadian oscillator revealed in individual cyanobacteria

Circadian oscillators, which provide internal daily periodicity, are found in a variety of living organisms, including mammals, insects, plants, fungi and cyanobacteria. Remarkably, these biochemical oscillators are resilient to external and internal modifications, such as temperature and cell division cycles. They have to be 'fluctuation (noise) resistant' because relative fluctuations in the number of messenger RNA and protein molecules forming the intracellular oscillators are likely to be large. In multicellular organisms, the strong temporal stability of circadian clocks, despite molecular fluctuations, can easily be explained by intercellular interactions. Here we study circadian rhythms and their stability in unicellular cyanobacteria Synechoccocus elongatus. Low-light-level microscopy has allowed us to measure gene expression under circadian control in single bacteria, showing that the circadian clock is indeed a property of individual cells. Our measurements show that the oscillators have a strong temporal stability with a correlation time of several months. In contrast to many circadian clocks in multicellular organisms, this stability seems to be ensured by the intracellular biochemical network, because the interactions between oscillators seem to be negligible.

Light control of hliA transcription and transcript stability in the cyanobacterium Synechococcus elongatus strain PCC 7942

The high-light-inducible proteins (HLIPs) of cyanobacteria are polypeptides involved in protecting the cells from high-intensity light (HL). The hliA gene encoding the HLIP from Synechococcus elongatus strain PCC 7942 is expressed in response to HL or low-intensity blue or UV-A light. In this study, we explore via Northern analysis details of the transcriptional regulation and transcript stability of the hliA gene under various light conditions. Transcript levels of the hliA gene increased dramatically upon a shift to HL or UV-A light to similar levels, followed by a rapid decrease in UV-A light, but not in HL, consistent with blue/UV-A light involvement in early stages of HL-mediated expression. A 3-min pulse of low-intensity UV-A light was enough to trigger hliA mRNA accumulation, indicating that a blue/UV-A photoreceptor is involved in upregulation of the gene. Low-intensity red light was found to cause a slight, transient increase in transcript levels (raising the possibility of red-light photoreceptor involvement), while light of other qualities had no apparent effect. No evidence was found for wavelength-specific attenuation of hliA transcript levels induced by HL or UV-A light. Transcript decay was slowed somewhat in darkness, and when photosynthetic electron transport was inhibited by darkness or treatment with DCMU, there appeared a smaller mRNA species that may represent a decay intermediate that accumulates when mRNA decay is slowed. Evidence suggests that upregulation of hliA by light is primarily a transcriptional response but conditions that cause ribosomes to stall on the transcript (e.g., a shift to darkness) can help stabilize hliA mRNA and affect expression levels.

Synechocystis 6803 mutants expressing distinct forms of the Photosystem II D1 protein from Synechococcus 7942: relationship between the psbA coding region and sensitivity to visible and UV-B radiation

Synechocystis PCC 6803 mutants expressing either the "low light" (D1:1) or the "high light" (D1:2) form of the Photosystem II (PSII) D1 protein from Synechococcus PCC 7942 were constructed and characterized with respect to properties of PSII and sensitivity to visible and UV-B radiation. The AI and AIII mutants (containing only the D1:1 and D1:2 forms, respectively) exhibited very similar PSII characteristics as the control strain and they differed only in the accelerated decay kinetics of flash-induced variable fluorescence measured in the presence of DCMU. However, the mutants showed increased sensitivity to photodamage induced by visible and UV-B radiation, with higher loss of PSII activity in the AI than in the AIII strain. Thus, the difference between strains containing D1:1 and D1:2 found previously in Synechococcus 7942 is maintained after transfer of corresponding psbA genes into Synechocystis 6803 and is directly related to the coding region of these genes. The higher light sensitivity of the AI mutant is caused partly by the higher rate of photodamage and partly by the less efficient PSII repair.

nblS, a gene involved in controlling photosynthesis-related gene expression during high light and nutrient stress in Synechococcus elongatus PCC 7942

The HliA protein of the cyanobacterium Synechococcus elongatus PCC 7942 is a small, thylakoid-associated protein that appears to play a role in photoprotection; its transcript rapidly accumulates in response to high-intensity light (HL) and the hli gene family is required for survival of cells in high light. In order to discover regulatory factors involved in HL acclimation in cyanobacteria, a screen was performed for chemically generated mutants unable to properly control expression of the hliA gene in response to HL. One such mutant was identified, and complementation analysis led to the identification of the affected gene, designated nblS. Based on its deduced protein sequence, NblS appears to be a membrane-bound, PAS-domain-bearing, sensor histidine kinase of two-component regulatory systems in bacteria. The nblS mutant was unable to properly control light intensity-mediated expression of several other photosynthesis-related genes, including all three psbA genes and the cpcBA genes. The mutant was also unable to control expression of the hliA and psbA genes in response to low-intensity blue/UV-A light, a response that may be related to the HL-mediated regulation of the genes. Additionally, in response to nutrient deprivation, the nblS mutant was unable to properly control accumulation of the nblA transcript and associated degradation of the light-harvesting phycobilisomes. The nblS mutant dies more rapidly than wild-type cells following exposure to HL or nutrient deprivation, likely due to its inability to properly acclimate to these stress conditions. Thus, the NblS protein is involved in the control of a number of processes critical for altering the photosynthetic apparatus in response to both HL and nutrient stress conditions.

Action spectrum of psbA gene transcription is similar to that of photoinhibition in Synechocystis sp. PCC 6803

The photosystem II (PSII) reaction center protein D1 undergoes rapid light-dependent turnover, which is caused by photoinhibition. To identify the photoreceptor(s) involved in the light-dependent expression of the psbA gene encoding the D1 protein, we determined the action spectra of psbA transcription, PSII activity, photosynthesis and photoinhibition in Synechocystis sp. PCC 6803. In accordance with its phycobilisome antenna, PSII showed the highest activity in the spectral region from yellow to red and only low activity in the ultraviolet-A (UV-A) to green region. Photoinhibition, in turn, was fastest in UV-A to violet light and a minor peak was found in the orange region. The action spectrum of psbA transcription resembled closely that of photoinhibition, suggesting that photoinhibition creates a signal for up-regulation of the psbA gene.

How do cyanobacteria sense and respond to light?

Cyanobacteria exhibit numerous responses to changes in the intensity and spectral quality of light. What sensors do cyanobacteria use to detect light and what are the mechanisms of signal transduction? The publication in 1996 of the complete genome sequence of the cyanobacterium Synechocystis 6803 provided a tremendous stimulus for research in this field, and many light-sensors and signal transducers have now been identified. However, our knowledge of cyanobacterial light-signal transduction remains fragmentary. This review summarizes what we know about the ways in which cyanobacteria perceive light, some of the ways which they respond to light signals and some recent achievements in elucidating the signal transduction mechanisms. Some problems in characterizing cyanobacterial signal transduction pathways are outlined and alternative experimental strategies are discussed.

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).

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.

Regulation of translation elongation in cyanobacteria: membrane targeting of the ribosome nascent-chain complexes controls the synthesis of D1 protein

The photosystem II reaction centre protein D1 is encoded by the psbA gene. The D1 protein is stable in darkness but undergoes rapid turnover in the light. Here, we show that, in cyanobacterium Synechocystis sp. PCC6803, the synthesis of the D1 protein is regulated at the level of translation elongation in addition to the previously known transcriptional regulation. When Synechocystis sp. PCC6803 cells were transferred from light to darkness, the psbA mRNA remained abundant for hours. Cytosolic ribosomes were attached to psbA transcripts in the dark, and translation continued up to a distinct pausing site. However, ribosome nascent D1 chain complexes were not targeted to the thylakoid membrane, and no full-length D1 protein was produced in darkness. The arrest in translation elongation was released in the light, concomitantly with targeting of ribosome D1 nascent-chain complexes to the thylakoid membrane, allowing the synthesis of the full-length D1 protein. Downregulation of membrane targeting of ribosome complexes was also observed in the light if damage to the D1 protein was slow. This novel type of regulation of prokaryotic translation functions to balance the synthesis and degradation of the rapidly turning over photosystem II D1 protein in Synechocystis sp. PCC6803.

Phytochrome A mediates blue light and UV-A-dependent chloroplast gene transcription in green leaves

We characterized the photobiology of light-activated chloroplast transcription and transcript abundance in mature primary leaves by using the following two systems: transplastomic promoter-reporter gene fusions in tobacco (Nicotiana tabacum), and phytochrome (phyA, phyB, and hy2) and cryptochrome (cry1) mutants of Arabidopsis. In both dicots, blue light and UV-A radiation were the major signals that activated total chloroplast and psbA, rbcL, and 16S rrn transcription. In contrast, transcription activities in plants exposed to red and far-red light were 30% to 85% less than in blue light/UV-A, depending on the gene and plant species. Total chloroplast, psbA, and 16S rrn transcription were 60% to 80% less in the Arabidopsis phyA mutant exposed to blue light/UV-A relative to wild type, thus definitively linking phyA signaling to these photoresponses. To our knowledge, the major role of phyA in mediating the blue light/UV-A photoresponses is a new function for phyA in chloroplast biogenesis at this stage of leaf development. Although rbcL expression in plants exposed to UV-A was 50% less in the phyA mutant relative to wild type, blue light-induced rbcL expression was not significantly affected in the phyA, phyB, and cry1 mutants. However, rbcL expression in blue light was 60% less in the phytochrome chromophore mutant, hy2, relative to wild type, indicating that another phytochrome species (phyC, D, or E) was involved in blue light-induced rbcL transcription. Therefore, at least two different phytochromes, as well as phytochrome-independent photosensory pathways, mediated blue light/UV-A-induced transcription of chloroplast genes in mature leaves.

Functional elements of the strong psbAI promoter of Synechococcus elongatus PCC 7942

The psbAI gene of the cyanobacterium Synechococcus elongatus PCC 7942 is one of three psbA genes that encode a critical photosystem II reaction center protein, D1. Regulation of the gene family in response to changes in the light environment is complex, occurs at transcriptional and posttranscriptional levels, and results in an interchange of two different forms of D1 in the membrane. Expression of psbAI is downregulated under high-intensity light (high light) in contrast to induction of the other two family members. We show that, in addition to a known accelerated degradation of the psbAI message, promoter activity decreases upon exposure to high light. Unlike the other psbA genes, additional sequences upstream of the psbAI -35 element are required for expression. Mutagenizing the atypical psbAI -10 element from TCTCCT to TATAAT increased the magnitude of expression from both psbAI::lacZ and psbAI::luxAB fusions but did not affect downregulation under high light. Inactivation of group 2 sigma factor genes rpoD2 and sigC, in both wild-type and -10-element mutagenized backgrounds, resulted in elevated psbAI::luxAB expression but did not alter the response to high light. The results are consistent with redundancy of promoter recognition among cyanobacterial group 2 sigma factors. Electrophoretic mobility shift assays showed that the DNA sequence corresponding to the untranslated leader of the psbAI message binds one or more proteins from an S. elongatus extract. The corresponding region of psbAII efficiently competed for this binding activity, suggesting a shared regulatory factor among these disparately regulated genes.

Expression of psbA genes is regulated at multiple levels in the cyanobacterium Synechococcus sp. PCC 7942

In cyanobacterium Synechococcus sp. PCC 7942 the photosystem II reaction-center protein D1 is encoded by three psbA genes. The psbAI gene encodes D1:1 protein, the form used for acclimated growth, and psbAII and psbAIII genes encode the stress-induced form, D1:2 protein. Strong light and low temperature have been shown to induce the expression of psbAII/III genes and down-regulate the expression of psbAI gene. Recently, we reported the involvement of reduced thiols in the up-regulation of psbAII/III genes. In this study, we have analyzed the regulation of psbA gene expression in Synechococcus further, at both the transcriptional and post-transcriptional levels. We show that the inhibitors of the photosynthetic electron-transfer chain, which have different effects on the redox state of the plastoquinone (PQ) pool, have similar effect on the transcription of psbA genes. The inhibitors 3-(3,4 dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) do not cause any changes in psbA gene expression when added under low-light conditions, but dramatically reduce the high-light induction of psbAII/III genes when added upon a high-light shift. Moreover, when the thiol reductant, dithiothreitol, is added to Synechococcus cells together with DCMU concomitant with the high-light shift, no inhibition of psbAII/III gene up-regulation takes place, indicating that the thiol redox state rather than the redox state of the PQ pool regulates psbA gene transcription. We also provide evidence for post-transcriptional regulation of psbA gene expression, particularly, inhibition of translation of psbAI transcripts at high light, and demonstrate that the D1 protein synthesis and degradation processes are coregulated in Synechococcus.

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.

Circadian programs in cyanobacteria: adaptiveness and mechanism

At least one group of prokaryotes is known to have circadian regulation of cellular activities--the cyanobacteria. Their "biological clock" orchestrates cellular events to occur in an optimal temporal program, and it can keep track of circadian time even when the cells are dividing more rapidly than once per day. Growth competition experiments demonstrate that the fitness of cyanobacteria is enhanced when the circadian period matches the period of the environmental cycle. Three genes have been identified that specifically affect circadian phenotypes. These genes, kaiA, kaiB, and kaiC, are adjacent to each other on the chromosome, thus forming a clock gene cluster. The clock gene products appear to interact with each other and form an autoregulatory feedback loop.

Expression of the psbA gene in the marine oxyphotobacteria Prochlorococcus spp

The oxygenic photosynthetic prokaryotes Prochlorococcus marinus SS120 (CCMP1375) and Prochlorococcus sp. MED4 (CCMP 1378) were previously shown to exhibit different pigmentation and ecophysiological characteristics. The former strain has a much lower divinyl-Chl a to b ratio and is adapted to lower photon flux densities than the latter. In contrast to the cyanobacteria examined so far, both strains possess only one copy of the psbA gene, encoding the D1 protein of photosystem II core. In acclimated steady-state cultures, psbA transcript levels were always higher at high irradiances in both strains. Upon a shift from low to high light, the psbA transcript levels increased in both strains but more quickly in MED4 than in SS120. They decreased during the opposite shift. Iron-starved MED4 cells overexpressed psbA at all assayed irradiances, suggesting that this species, representative of populations from naturally iron-depleted oceanic areas, may have developed a particular compensation mechanism. The similar effects of DCMU and DBMIB on the expression of psbA suggest that light regulation of psbA in Prochlorococcus may be mediated by the electron transport chain. The energy state of cells could, however, also be involved in this regulation, since cultures of both strains subjected to darkness showed psbA levels significantly lower when glucose was added.

UV-B-induced differential transcription of psbA genes encoding the D1 protein of photosystem II in the Cyanobacterium synechocystis 6803

UV-B irradiation of intact Synechocystis sp. PCC 6803 cells results in the loss of photosystem II activity, which can be repaired via de novo synthesis of the D1 (and D2) reaction center subunits. In this study, we investigated the effect of UV-B irradiation on the transcription of the psbA2 and psbA3 genes encoding identical D1 proteins. We show that UV-B irradiation increases the level of psbA2 mRNA 2-3-fold and, more dramatically, it induces a 20-30-fold increase in the accumulation of the psbA3 mRNA even at levels of irradiation too low to produce losses of either photosystem II activity or D1 protein. The induction of psbA3 transcript accumulation is specific for UV-B light (290-330 nm). Low intensity UV-A emission (330-390 nm) and white light induce only a small, at most, 2-3-fold enhancement, whereas no effect of blue light was observed. Expression patterns of chimeric genes containing the promoter regions of the psbA2, psbA3 genes fused to the firefly luciferase (luc) reporter gene indicate that (i) transcription of psbA2/luc and psbA3/luc transgenes was elevated, similarly to that of the endogenous psbA genes, by UV-B irradiation, and that (ii) a short, 80-base pair psbA3 promoter fragment is sufficient to maintain UV-B-induced transcription of the luc reporter gene. Furthermore, our findings indicate that UV-B-induced expression of the psbA2 and psbA3 genes is a defense response against UV-B stress, which is regulated, at least, partially at the level of transcription and does not require active electron transport.

The cyanobacterium Synechococcus resists UV-B by exchanging photosystem II reaction-center D1 proteins

Current ambient UV-B levels can significantly depress productivity in aquatic habitats, largely because UV-B inhibits several steps of photosynthesis, including the photooxidation of water catalyzed by photosystem II. We show that upon UV-B exposure the cyanobacterium Synechococcus sp. PCC 7942 rapidly changes the expression of a family of three psbA genes encoding photosystem II D1 proteins. In wild-type cells the psbAI gene is expressed constitutively, but strong accumulations of psbAII and psbAIII transcripts are induced within 15 min of moderate UV-B exposure (0.4 W/m2). This transcriptional response causes an exchange of two distinct photosystem II D1 proteins. D1:1 is encoded by psbAI, but on UV-B exposure, it is largely replaced by the alternate D1:2 form, encoded by both psbAII and psbAIII. The total content of D1 and other photosystem II reaction center protein, D2, remained unchanged throughout the UV exposure, as did the content and composition of the phycobilisome. Wild-type cells suffered only slight transient inhibition of photosystem II function under UV-B exposure. In marked contrast, under the same UV-B treatment, a mutant strain expressing only psbAI suffered severe (40%) and sustained inhibition of photosystem II function. Another mutant strain with constitutive expression of psbAII and psbAIII was almost completely resistant to the UV-B treatment, showing no inhibition of photosystem II function and only a slight drop in electron transport. In Synechococcus the rapid exchange of alternate D1 forms, therefore, accounts for much of the cellular resistance to UV-B inhibition of photosystem II activity and photosynthetic electron transport. This molecular plasticity may be an important element in community-level responses to UV-B, where susceptibility to UV-B inhibition of photosynthesis changes diurnally.

CYANOBACTERIAL CIRCADIAN RHYTHMS

Evidence from a number of laboratories over the past 12 years has established that cyanobacteria, a group of photosynthetic eubacteria, possess a circadian pacemaker that controls metabolic and genetic functions. The cyanobacterial circadian clock exhibits the three intrinsic properties that have come to define the clocks of eukaryotes: The timekeeping mechanism controls rhythms that show a period of about 24 h in the absence of external signals, the phase of the rhythms can be reset by light/dark cues, and the period is relatively insensitive to temperature. The promise of cyanobacteria as simple models for elucidating the biological clock mechanism is being fulfilled, as mutants affected in period, rhythm generation, and rhythm amplitude, isolated through the use of real time reporters of gene expression, have implicated genes involved in these aspects of the clock.

Leaf development and phytochrome modulate the activation ofpsbD-psbC transcription by high-fluence blue light in barley chloroplasts

Activation ofpsbD transcription by light assists in maintaining the synthesis of the PS II reaction center protein, D2, which is photodamaged in plants exposed to high light. In this study, the photosensory pathways and mechanisms that regulate the expression of thepsbD-psbC light-responsive promoter, LRP, were investigated during barley (Hordeum vulgare L.) seedling development. Accumulation ofpsbD-psbC mRNAs in response to light was observed in apical sections of primary leaves with little or no increase in mRNAs in basal sections. In both 4.5- and 7.5-day-old etiolated seedlings, blue light was most effective for activating mRNA accumulation from thepsbD-psbC LRP. However, the response of the LRP to red light increased 7-fold in 7.5-day relative to 4.5-day-old seedlings. Blue light preferentially activatedpsbD-psbC transcription, while red light was most effective for activating total plastid transcription and the expression of genes encoding the small (RbcS) and large (rbcL) subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase and Chl-a/b-binding protein (Lhcb). The stimulatory effects of red light onpsbD-psbC expression were partially reversed, and of blue light were not reversed, by subsequent pulses of far-red light. In contrast, continuous far-red light given together with blue light enhancedpsbD-psbC transcription in a synergistic manner. These observations indicate that phytochrome modulates the effects of high-fluence blue light onpsbD-psbC transcription by affecting total plastid transcription.

Two forms of the Photosystem II D1 protein alter energy dissipation and state transitions in the cyanobacterium Synechococcus sp. PCC 7942

Synechococcus sp. PCC 7942 (Anacystis nidulans R2) contains two forms of the Photosystem II reaction centre protein D1, which differ in 25 of 360 amino acids. D1: 1 predominates under low light but is transiently replaced by D1:2 upon shifts to higher light. Mutant cells containing only D1:1 have lower photochemical energy capture efficiency and decreased resistance to photoinhibition, compared to cells containing D1:2. We show that when dark-adapted or under low to moderate light, cells with D1:1 have higher non-photochemical quenching of PS II fluorescence (higher qN) than do cells with D1:2. This is reflected in the 77 K chlorophyll emission spectra, with lower Photosystem II fluorescence at 697-698 nm in cells containing D1:1 than in cells with D1:2. This difference in quenching of Photosystem II fluorescence occurs upon excitation of both chlorophyll at 435 nm and phycobilisomes at 570 nm. Measurement of time-resolved room temperature fluorescence shows that Photosystem II fluorescence related to charge stabilization is quenched more rapidly in cells containing D1:1 than in those with D1:2. Cells containing D1:1 appear generally shifted towards State II, with PS II down-regulated, while cells with D1:2 tend towards State I. In these cyanobacteria electron transport away from PS II remains non-saturated even under photoinhibitory levels of light. Therefore, the higher activity of D1:2 Photosystem II centres may allow more rapid photochemical dissipation of excess energy into the electron transport chain. D1:1 confers capacity for extreme State II which may be of benefit under low and variable light.

The cyanobacterium Synechococcus modulates Photosystem II function in response to excitation stress through D1 exchange

In this minireview we discuss effects of excitation stress on the molecular organization and function of PS II as induced by high light or low temperature in the cyanobacterium Synechococcus sp. PCC 7942. Synechococcus displays PS II plasticity by transiently replacing the constitutive D1 form (D1:1) with another form (D1:2) upon exposure to excitation stress. The cells thereby counteract photoinhibition by increasing D1 turn over and modulating PS II function. A comparison between the cyanobacterium Synechococcus and plants shows that in cyanobacteria, with their large phycobilisomes, resistance to photoinhibition is mainly through the dynamic properties (D1 turnover and quenching) of the reaction centre. In contrast, plants use antenna quenching in the light-harvesting complex as an important means to protect the reaction center from excessive excitation.

The grand design of photosynthesis: Acclimation of the photosynthetic apparatus to environmental cues

Dynamic acclimation of the photosynthetic apparatus in response to environmental cues, particularly light quantity and quality, is a widely-observed and important phenomenon which contributes to the tolerance of plants against stress and helps to maintain, as far as possible, optimal photosynthetic efficiency and resource utilization. This mini-review represents a scrutiny of a number of possible photoreceptors (including the two photosystems acting as light sensors) and signal transducers that may be involved in producing acclimation responses. We suggest that regulation by signal transduction may be effected at each of several possible points, and that there are multiple regulatory mechanisms for photosynthetic acclimation.

Specific binding of Synechococcus sp. strain PCC 7942 proteins to the enhancer element of psbAII required for high-light-induced expression

The psbAII gene of the cyanobacterium Synechococcus sp. strain PCC 7942 is a member of a three-gene family that encodes the D1 protein of the photosystem II reaction center. Transcription of psbAII is rapidly induced when the light intensity reaching the culture increases from 125 microE.m-2.s-1 (low light) to 750 microE.m-2.s-1 (high light). The DNA segment upstream of psbAII that corresponds to the untranslated leader of its major transcript has enhancer activity and confers high-light induction. We show that one or more soluble proteins from PCC 7942 specifically bind to this region of psbAII (designated the enhancer element). In vivo footprinting showed protein binding to the enhancer element in high-light-exposed cell samples but not in those maintained at low light, even though in vitro mobility shifts were detectable with extracts from low- or high-light-grown cells. When 12 bp were deleted from the psbAII enhancer element, protein binding was impaired and high-light induction of both transcriptional and translational psbAII-lacZ reporters was significantly reduced. This finding indicates that protein binding to this region is required for high-light induction of psbAII. The mutant element also showed impaired enhancer activity when combined with a heterologous promoter.

Form II of D1 is important during transition from standard to high light intensity in Synechococcus sp. strain PCC 7942

In Synechococcus sp. strain PCC 7942 the D1 protein of Photosystem II is encoded by a multigene family; psbAI encodes Form I of D1 whereas both psbAII and psbAIII encode Form II. The psbA genes are differentially regulated in response to changes in light intensity, such that psbAI expression and Form I predominate at standard light intensity, whereas psbAII and psbAIII are induced at high light intensity, causing insertion of Form II into the thylakoids. The present study addressed whether high-light induced Form II is important for Synechococcus cells during adaptation to high light intensity. Wild-type Synechococcus, and mutants which produce only Form I (R2S2C3) or only Form II (R2K1), were co-cultured at standard light (130 μE · m(-2) · s(-1)) and then shifted to high light (750 μE·m(-2)·s(-1)). Measurement of the proportion of each cell type at various time intervals revealed that the growth of R2S2C3, which has psbAII and psbAIII inactive, and thus lacks Form II, is transiently impaired upon shift to high light. Both mutants R2S2C3 and R2K1 maintained normal levels of psbA messages and D1 protein under standard and high light through an unknown mechanism that compensates for the inactive psbA genes. Thus, the impairment of R2S2C3 at high light is not due to a deficiency of D1 protein, but results from lack of Form II. We discounted the influence of possible secondary mutations by re-creating the psbA-inactivated mutants and testing the newly isolated strains. We conclude that Form II of D1 is intrinsically important for Synechococcus cells during a critical transition period after exposure to high light intensities.