Circadian expression of the carbonic anhydrase gene, Cah1, in Chlamydomonas reinhardtii

Carbonic anhydrases CA1 and CA4 function in atmospheric CO2-modulated disease resistance

Carbonic anhydrases CA1 and CA4 attenuate plant immunity and can contribute to altered disease resistance levels in response to changing atmospheric CO₂ conditions. β-Carbonic anhydrases (CAs) play an important role in CO₂ metabolism and plant development, but have also been implicated in plant immunity. Here we show that the bacterial pathogen Pseudomonas syringae and application of the microbe-associated molecular pattern (MAMP) flg22 repress CA1 and CA4 gene expression in Arabidopsis thaliana. Using the CA double-mutant ca1ca4, we provide evidence that CA1 and CA4 play an attenuating role in pathogen- and flg22-triggered immune responses. In line with this, ca1ca4 plants exhibited enhanced resistance against P. syringae, which was accompanied by an increased expression of the defense-related genes FRK1 and ICS1. Under low atmospheric CO₂ conditions (150 ppm), when CA activity is typically low, the levels of CA1 transcription and resistance to P. syringae in wild-type Col-0 were similar to those observed in ca1ca4. However, under ambient (400 ppm) and elevated (800 ppm) atmospheric CO₂ conditions, CA1 transcription was enhanced and resistance to P. syringae reduced. Together, these results suggest that CA1 and CA4 attenuate plant immunity and that differential CA gene expression in response to changing atmospheric CO₂ conditions contribute to altered disease resistance levels.

Expression of seven carbonic anhydrases in red alga Gracilariopsis chorda and their subcellular localization in a heterologous system, Arabidopsis thaliana

Red alga, Gracilariopsis chorda, contains seven carbonic anhydrases that can be grouped into α-, β- and γ-classes. Carbonic anhydrases (CAHs) are metalloenzymes that catalyze the reversible hydration of CO₂. These enzymes are present in all living organisms and play roles in various cellular processes, including photosynthesis. In this study, we identified seven CAH genes (GcCAHs) from the genome sequence of the red alga Gracilariopsis chorda and characterized them at the molecular, cellular and biochemical levels. Based on sequence analysis, these seven isoforms were categorized into four α-class, one β-class, and two γ-class isoforms. RNA sequencing revealed that of the seven CAHs isoforms, six genes were expressed in G. chorda in light at room temperature. In silico analysis revealed that these seven isoforms localized to multiple subcellular locations such as the ER, mitochondria and cytosol. When expressed as green fluorescent protein fusions in protoplasts of Arabidopsis thaliana leaf cells, these seven isoforms showed multiple localization patterns. The four α-class GcCAHs with an N-terminal hydrophobic leader sequence localized to the ER and two of them were further targeted to the vacuole. GcCAHβ1 with no noticeable signal sequence localized to the cytosol. The two γ-class GcCAHs also localized to the cytosol, despite the presence of a predicted presequence. Based on these results, we propose that the red alga G. chorda also employs multiple CAH isoforms for various cellular processes such as photosynthesis.

Coordination between photorespiration and carbon concentrating mechanism in Chlamydomonas reinhardtii: transcript and protein changes during light-dark diurnal cycles and mixotrophy conditions

Carbon concentrating mechanism (CCM) and photorespiration (PR) are interlinked and co-regulated in Chlamydomonas reinhardtii, but conditions where co-regulation alters are not sufficiently explored. Here, we uncover that PR gene transcripts, like CCM transcripts, are induced even in the dark when both processes are not active. Such diurnal cycles show that transcript levels peak in the middle of 12 h day, decline by early part of 12-h dark followed by their onset again at mid-dark. Interestingly, the onset in the mid-dark phase is sensitive to high CO₂, implying that the active carbon sensing mechanism operates even in the dark. The rhythmic alterations of both CCM and PR transcript levels are unlinked to circadian clock: the "free-running state" reveals no discernible rhythmicity in transcript changes. Only continuous light leads to high transcript levels but no detectable transcripts were observed in continuous dark. Asynchronous continuous light cultures, upon shifting to low from high CO₂ exhibit only transient induction of PR transcripts/proteins while CCM transcript induction is stable, indicating the loss of co-regulation between PR and CCM gene transcription. Lastly, we also describe that both CCM and PR transcripts/proteins are induced in low CO₂ even in mixotrophic cultures, but only in high light, the same being attenuated in high CO₂, implying that high light is a mandatory "trigger" for CCM and PR induction in low CO₂ mixotrophy. Our study provides comprehensive analyses of conditions where CCM and PR were differently regulated, setting a paradigm for a detailed mechanistic probing of these responses.

Isolation, phenotypic characterization and genome wide analysis of a Chlamydomonas reinhardtii strain naturally modified under laboratory conditions: towards enhanced microalgal biomass and lipid production for biofuels

BACKGROUND

Microalgal strain development through genetic engineering has received much attention as a way to improve the traits of microalgae suitable for biofuel production. However, there are still some limitations in application of genetically modified organisms. In this regard, there has been recent interest in the isolation and characterization of superior strains naturally modified and/or adapted under a certain condition and on the interpretation of phenotypic changes through the whole genome sequencing.

RESULTS

In this study, we isolated and characterized a novel derivative of C. reinhardtii, whose phenotypic traits diverged significantly from its ancestral strain, C. reinhardtii CC-124. This strain, designated as CC-124H, displayed cell population containing increased numbers of larger cells, which resulted in an increased biomass productivity compared to its ancestor CC-124. CC-124H was further compared with the CC-124 wild-type strain which underwent long-term storage under low light condition, designated as CC-124L. In an effort to evaluate the potential of CC-124H for biofuel production, we also found that CC-124H accumulated 116 and 66% greater lipids than that of the CC-124L, after 4 days under nitrogen and sulfur depleted conditions, respectively. Taken together, our results revealed that CC-124H had significantly increased fatty acid methyl ester (FAME) yields that were 2.66 and 1.98 times higher than that of the CC-124L at 4 days after the onset of cultivation under N and S depleted conditions, respectively, and these higher FAME yields were still maintained by day 8. We next analyzed single nucleotide polymorphisms (SNPs) and insertion/deletions (indels) based on the whole genome sequencing. The result revealed that of the 44 CDS region alterations, 34 resulted in non-synonymous substitutions within 33 genes which may mostly be involved in cell cycle, division or proliferation.

CONCLUSION

Our phenotypic analysis, which emphasized lipid productivity, clearly revealed that CC-124H had a dramatically enhanced biomass and lipid content compared to the CC-124L. Moreover, SNPs and indels analysis enabled us to identify 34 of non-synonymous substitutions which may result in phenotypic changes of CC-124H. All of these results suggest that the concept of adaptive evolution combined with genome wide analysis can be applied to microalgal strain development for biofuel production.

The CO2 concentrating mechanism and photosynthetic carbon assimilation in limiting CO2 : how Chlamydomonas works against the gradient

The CO2 concentrating mechanism (CCM) represents an effective strategy for carbon acquisition that enables microalgae to survive and proliferate when the CO2 concentration limits photosynthesis. The CCM improves photosynthetic performance by raising the CO2 concentration at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), simultaneously enhancing carbon fixation and suppressing photorespiration. Active inorganic carbon (Ci) uptake, Rubisco sequestration and interconversion between different Ci species catalyzed by carbonic anhydrases (CAs) are key components in the CCM, and an array of molecular regulatory elements is present to facilitate the sensing of CO2 availability, to regulate the expression of the CCM and to coordinate interplay between photosynthetic carbon metabolism and other metabolic processes in response to limiting CO2 conditions. This review intends to integrate our current understanding of the eukaryotic algal CCM and its interaction with carbon assimilation, based largely on Chlamydomonas as a model, and to illustrate how Chlamydomonas acclimates to limiting CO2 conditions and how its CCM is regulated.

Dynamics of carbon-concentrating mechanism induction and protein relocalization during the dark-to-light transition in synchronized Chlamydomonas reinhardtii

In the model green alga Chlamydomonas reinhardtii, a carbon-concentrating mechanism (CCM) is induced under low CO2 in the light and comprises active inorganic carbon transport components, carbonic anhydrases, and aggregation of Rubisco in the chloroplast pyrenoid. Previous studies have focused predominantly on asynchronous cultures of cells grown under low versus high CO2. Here, we have investigated the dynamics of CCM activation in synchronized cells grown in dark/light cycles compared with induction under low CO2. The specific focus was to undertake detailed time course experiments comparing physiology and gene expression during the dark-to-light transition. First, the CCM could be fully induced 1 h before dawn, as measured by the photosynthetic affinity for inorganic carbon. This occurred in advance of maximum gene transcription and protein accumulation and contrasted with the coordinated induction observed under low CO2. Between 2 and 1 h before dawn, the proportion of Rubisco and the thylakoid lumen carbonic anhydrase in the pyrenoid rose substantially, coincident with increased CCM activity. Thus, other mechanisms are likely to activate the CCM before dawn, independent of gene transcription of known CCM components. Furthermore, this study highlights the value of using synchronized cells during the dark-to-light transition as an alternative means of investigating CCM induction.

Dehydroascorbate: a possible surveillance molecule of oxidative stress and programmed cell death in the green alga Chlamydomonas reinhardtii

Chlamydomonas reinhardtii tolerates relatively high H2 O2 levels that induce an array of antioxidant activities. However, rather than rendering the cells more resistant to oxidative stress, the cells become far more sensitive to an additional H2 O2 dose. If H2 O2 is provided 1.5-9 h after an initial dose, it induces programmed cell death (PCD) in the wild-type, but not in the dum1 mutant impaired in the mitochondrial respiratory complex III. This mutant does not exhibit a secondary oxidative burst 4-5 h after the inducing H2 O2 , nor does it activate metacaspase-1 after the second H2 O2 treatment. The intracellular dehydroascorbate level, a product of ascorbate peroxidase, increases under conditions leading to PCD. The addition of dehydroascorbate induces PCD in the wild-type and dum1 cultures, but higher levels are required in dum1 cells, where it is metabolized faster. The application of dehydroascorbate induces the expression of metacaspase-2, which is much stronger than the expression of metacaspase-1. The presence or absence of oxidative stress, in addition to the rise in internal dehydroascorbate, may determine which metacaspase is activated during Chlamydomonas PCD. Cell death is strongly affected by the timing of H2 O2 or dehydroascorbate admission to synchronously grown cultures, suggesting that the cell cycle phase may distinguish cells that perish from those that do not.

Acclimation to low or limiting CO2 in non-synchronous Chlamydomonas causes a transient synchronization of the cell division cycle

Acclimation of Chlamydomonas reinhardtii (hereafter, Chlamydomonas) to low or limiting CO(2) or inorganic carbon (C(i)) has been studied fairly extensively with regard to the mechanisms underlying the inducible C(i) acquisition systems and the signal transduction pathway involved in recognizing and responding to decreased C(i) availability. Investigation of low C(i )acclimation responses typically is performed with non-synchronous cultures grown in continuous light to avoid any effects of the cell division cycle (CDC) confounding interpretation of acclimation responses. However, little is known about whether acclimation to low C(i) might affect the distribution of cells among the various stages of the CDC. To investigate the effects of a limiting-C(i) challenge on the CDC of Chlamydomonas, flow cytometry was used to monitor the distribution of cells among the CDC stages in both synchronous and non-synchronous cultures during acclimation to low or limiting C(i). When faced with C(i) limitation, non-synchronous cultures of Chlamydomonas undergo transient synchronization as those cells past the Commitment point of the CDC undergo division, while the remainder of the cells pause their growth in early G-phase, with the result that the cells all accumulate in early G-phase, appearing transiently synchronized until acclimated sufficiently to the decreased C(i) for growth to resume. This perturbation of the CDC by a limiting-C(i) challenge has important implications for the interpretation of gene expression and other responses apparently induced by low or limiting C(i).

Regulation of the expression of H43/Fea1 by multi-signals

The composition of extracellular proteins is known to be drastically changed in the unicellular green alga Chlamydomonas reinhardtii when the cells are transferred from ambient CO(2) to elevated CO(2) conditions. We previously observed very high production of the H43/Fea1 protein under high-CO(2) (0.3-3% in air) conditions. In addition, H43/Fea1 gene expression was reported to be induced under iron-deficient and cadmium-excess conditions, but it remains unclear how gene expression is regulated by multiple signals. To elucidate the regulatory mechanism of H43/Fea1 expression, this study intended to identify a high-CO(2)-responsive cis-element in a wall-deficient strain C. reinhardtti CC-400. Cells incubated in the presence of acetate in the dark, namely heterotrophically generated high-CO(2) conditions, were used for inducing H43/Fea1 gene expression following our previous study (Hanawa et al., Plant Cell Physiol 48:299-309, 2007) in Fe-sufficient and Cd-deficient medium to prevent the generation of other signals. First, we constructed a reporter assay system using transformants constructed by introducing genes with series of 5'-deleted upstream sequences of H43/Fea1 that were fused to a coding sequence of the Ars for arylsulfatase2 reporter gene. Consequently, the high-CO(2)-responsive cis-element (HCRE) was found to be located at a -537/-370 upstream region from the transcriptional initiation site of H43/Fea1. However, it still remains possible that a -724/-537 upstream region may also have a significant role in activating gene expression regulated by high-CO(2). Remarkably, a -925/-370 upstream region could successfully activate the Ars reporter gene under heterotrophically generated high-CO(2) conditions even when the sequence containing two Fe-deficiency-responsive elements was completely deleted. These results clearly showed that H43/Fea1 expression is regulated by high-CO(2) signal independently via the HCRE that is located distantly from Fe-deficient-signal responsive element, indicating that H43/Fea1 is a multi-signal-regulated gene.

Effects of granule-bound starch synthase I-defective mutation on the morphology and structure of pyrenoidal starch in Chlamydomonas

Lowering of the CO₂ concentration in the environment induces development of a pyrenoidal starch sheath, as well as that of pyrenoid and CO₂-concentrating mechanisms, in many microalgae. In the green algae Chlamydomonas and Chlorella, activity of granule-bound starch synthase (GBSS) concomitantly increases under these conditions. In this study, effects of the GBSS-defective mutation (sta2) on the development of pyrenoidal starch were investigated in Chlamydomonas. Stroma starch- and pyrenoid starch-enriched samples were obtained from log-phase cells grown with air containing 5% CO₂ (high-CO₂ conditions favouring stromal starch synthesis) and from those transferred to low-CO₂ conditions (air level, 0.04% CO₂, favouring pyrenoidal starch synthesis) for 6h, respectively. In the wild type, total starch content per culture volume did not increase during the low-CO₂ conditions, in spite of the development of pyrenoidal starch, suggesting that degradation of some part of stroma starch and synthesis of pyrenoid starch simultaneously occur under these conditions. Even in the GBSS-deficient mutants, pyrenoid and pyrenoid starch enlarged after lowering of the CO₂ concentration. However, the morphology of the pyrenoid starch was thinner and more fragile than the wild type, suggesting that GBSS does affect the morphology of pyrenoidal starch. Surprisingly normal GBSS activity is shown to be required to obtain the high A-type crystallinity levels that we now report for pyrenoidal starch. A model is presented explaining how GBSS-induced starch granule fusion may facilitate the formation of the pyrenoidal starch sheath.

Granule-bound starch synthase cDNA in Chlorella kessleri 11 h: cloning and regulation of expression by CO(2) concentration

The cDNA for the granule-bound starch synthase (GBSS; ADP-glucose-starch glucosyltransferase, EC 2.4.1.21) of Chlorella kessleri 11 h was isolated and characterized. CkGBSS encodes a 609-amino acid polypeptide (65,627 Da) that includes an N-terminal hydrophobic signal peptide of 55 amino acids. The deduced amino acid sequence of the mature CkGBSS polypeptide shares a greater identity (65%) to that of the GBSS protein of Chlamydomonas reinhardtii, than to those of vascular plant species, but does not have the extra-long C-terminal sequence found in C. reinhardtii. When CO(2 )concentration was decreased from 3 to 0.04% (air level) in light, the levels of CkGBSS mRNA, CkGBSS protein, and GBSS activity increased. Under this condition, pyrenoid and pyrenoid starch developed, and the relative amount of amylose in starch increased. These observations suggest that low CO(2) level up-regulates GBSS biosynthesis at the transcriptional level.

Regulation of the expression of intracellular beta-carbonic anhydrase in response to CO2 and light in the marine diatom Phaeodactylum tricornutum

Cells of the marine diatom Phaeodactylum tricornutum Bohlin (UTEX 642) grown in 5% CO(2) were transferred to air-level CO(2) in the light or dark and allowed to acclimate to air. No accumulation of the transcript of the P. tricornutum beta-carbonic anhydrase 1 (ptca1) was detected in 5% CO(2)-grown cells, but ptca1 mRNA accumulated and reached a peak after 6 h acclimation to air but decreased over the next 18 h. A similar accumulation time course was observed in cells air-acclimated in the dark, except that levels of mRNA were <50% those in the light. These results suggest that air-level [CO(2)] is required to trigger the transcription of ptca1 and that light affects the extent of acclimation. During acclimation to air for 120 h in the light, levels of ptca1 mRNA exhibited a periodic oscillation with a cycle of about 24 h, which, however, was not reflected in protein accumulation levels. A 5'-upstream region from the transcription-start site toward -1,292 bp of ptca1 was cloned by inverse polymerase chain reaction, and 5'-truncations were carried out on this fragment. The truncated promoter regions were fused with the beta-glucuronidase gene (uidA) and introduced into P. tricornutum. The promoter fragments, truncated at positions -1,292, -824, -484, -225, and -70 bp, conferred on transformants clear CO(2)-responsive beta-glucuronidase expressions. In contrast, the CO(2)-responsive regulation was severely impaired or completely abolished by truncations, respectively, at position -50 or -30 bp. These results indicate that critical cis-elements required for CO(2)-responsive transcription of ptca1 may be located between -70 and -30 bp relative to the transcription start site.

Identification of novel clock-controlled genes by cDNA macroarray analysis in Chlamydomonas reinhardtii

Circadian rhythms are self-sustaining oscillations whose period length under constant conditions is about 24 h. Circadian rhythms are widespread and involve functions as diverse as human sleep-wake cycles and cyanobacterial nitrogen fixation. In spite of a long research history, knowledge about clock-controlled genes is limited in Chlamydomonas reinhardtii. Using a cDNA macroarray containing 10 368 nuclear-encoded genes, we examined global circadian regulation of transcription in Chlamydomonas. We identified 269 candidates for circadianly expressed gene. Northern blot analysis confirmed reproducible and sustainable rhythmicity for 12 genes. Most genes exhibited peak expression at the transition point between day and night. One hundred and eighteen genes were assigned predicted annotations. The functions of the cycling genes were diverse and included photosynthesis, respiration, cellular structure, and various metabolic pathways. Surprisingly, 18 genes encoding chloroplast ribosomal proteins showed a coordinated circadian pattern of expression and peaked just at the beginning of subjective day. The co-regulation of genes bearing a similar function was also observed in genes involved in cellular structure. They peaked at the end of the subjective night, which is when the regeneration of cell walls and flagella in daughter cells occurs. Expression of the chlamyopsin gene, which encodes an opsin-type photoreceptor, also exhibited circadian rhythm.

The novel Myb transcription factor LCR1 regulates the CO2-responsive gene Cah1, encoding a periplasmic carbonic anhydrase in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii acclimates to CO2-limiting stress by inducing a set of genes for a carbon-concentrating mechanism (CCM). This set includes the gene Cah1, which encodes a periplasmic carbonic anhydrase. Although physiological aspects of CO2response have been extensively studied, regulatory components, such as transcription factors involved in the acclimation, have not been well described in eukaryotic microalgae. Using an arylsulfatase gene driven by the Cah1 promoter, a regulatory mutant of Cah1 was isolated and named lcr1 (for low-CO2 stress response). The photosynthetic affinity for inorganic carbon of lcr1 was reduced compared with that of wild-type cells. Expression of three low-CO2-inducible genes, Cah1, Lci1, and Lci6, were regulated by LCR1 as shown by cDNA array and RNA gel blot analyses. The Lcr1 gene encodes a protein of 602 amino acids containing a single Myb domain, which binds to the Cah1-promoter region. Expression of Lcr1 was induced by lowering CO2 levels and controlled by the regulatory factor CCM1. These results suggest that LCR1 transmits the low CO2 signal to at least three CO2-responsive genes and then fully induces CCM.

Chlamydomonas reinhardtii as a unicellular model for circadian rhythm analysis

Chlamydomonas reinhardtii has been used as an experimental model organism for circadian rhythm research for more than 30 yr. Some of the physiological rhythms of this alga are well established, and several clock mutants have been isolated. The cloning of clock genes from these mutant strains by positional cloning is under way and should give new insights into the mechanism of the circadian clock. In a spectacular space experiment, the question of the existence of an endogenous clock vs. an exogenous mechanism has been studied in this organism. With the emergence of molecular analysis of circadian rhythms in plants in 1985, a circadian gene expression pattern of several nuclear and chloroplast genes was detected. Evidence is now accumulating that shows circadian control at the translational level. In addition, the gating of the cell cycle by the circadian clock has been analyzed. This review focuses on the different aspects of circadian rhythm research in C. reinhardtii over the past 30 yr. The suitability of Chlamydomonas as a model system in chronobiology research and the adaptive significance of the observed rhythms will be discussed.

Circadian rhythms in microalgae

Circadian rhythms have been described in a variety of microalgae. In each group, some model organisms arose and most detailed studies have been done with them. They include the cyanobacterium ("blue-green alga") Synechococcus and eukaryotic microalgae Gonyaulax polyedra (Dinophyta), Chlamydomonas reinhardtii (Chlorophyta), and Euglena gracilis (Euglenophyta). This review focuses on recent approaches to depict molecular components of the circadian system and the mechanisms of regulation in these organisms. In Synechococcus, the identification of the kailocus, which represents a central part of its oscillatory system, is discussed, as well as diverse approaches based on a luminescent reporter gene, which is driven by a clock-controlled cyanobacterial promoter. In eukaryotic microalgae, the diversity of genes/proteins that are controlled by the circadian clock is described and the kind of regulation (transcriptional and translational control) is emphasized. The role and function of conserved clock-controlled RNA-binding proteins such as CCTR from Gonyaulaxor Chlamy 1 from Chlamydomonas are discussed.

CHLAMYDOMONAS AS A MODEL ORGANISM

The unicellular green alga Chlamydomonas offers a simple life cycle, easy isolation of mutants, and a growing array of tools and techniques for molecular genetic studies. Among the principal areas of current investigation using this model system are flagellar structure and function, genetics of basal bodies (centrioles), chloroplast biogenesis, photosynthesis, light perception, cell-cell recognition, and cell cycle control. A genome project has begun with compilation of expressed sequence tag data and gene expression studies and will lead to a complete genome sequence. Resources available to the research community include wild-type and mutant strains, plasmid constructs for transformation studies, and a comprehensive on-line database.

Fluorescent differential display identifies circadian clock-regulated genes in Arabidopsis thaliana

Circadian rhythms in gene expression were first observed in plants more than 13 years ago, but the underlying mechanism controlling rhythmic gene expression is still not understood. The isolation of novel circadian clock-controlled genes (ccgs) is likely to provide new tools for studying circadian rhythms. Fluorescent differential display (FDD) was used to screen Arabidopsis thaliana mRNAs for cycling transcripts. Seventy PCR primer pairs were screened, and 17 different cycling bands were observed out of an estimated 10,500 bands screened. The identities of 10 bands were determined, and the rhythmic gene expression was confirmed using northern blot analysis. The 10 cycling bands represent 7 different genes, 6 of which are present in the databases and 1 that does not match anything in current databases. The rhythmic expression of the 7 genes is composed of four distinct phases of clock regulation. The results demonstrate that FDD can be used to isolate ccgs. The genes identified in this screen range from known A. thaliana ccgs, as well as genes shown to be clock controlled in other plant species, to a novel gene that may encode a pioneer protein. Further study of these ccgs is likely to increase our understanding of circadian-regulated gene expression.

CO(2)-responsive transcriptional regulation of CAH1 encoding carbonic anhydrase is mediated by enhancer and silencer regions in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii adapts to the stress of CO(2)-limiting conditions through the induction of a set of genes including CAH1, which encodes a periplasmic carbonic anhydrase. CAH1 is up-regulated under low-CO(2) conditions (air containing 0.04% [v/v] CO(2)) in the presence of light, whereas it is down-regulated under high-CO(2) conditions (5% [v/v] CO(2)) or in the dark. In an effort to identify cis-elements involved in the transcriptional regulation of CAH1, a series of 5'-nested deletions of the region upstream of CAH1 were fused to a promoterless arylsulfatase reporter gene (ARS). The upstream region from -651 to +41 relative to the transcription start site was sufficient to regulate the expression of ARS with kinetics similar to those of endogenous CAH1. Deletion of the region between -651 and -294 resulted in a significant decrease in the level of arylsulfatase activity expressed under low-CO(2) conditions. The 543-bp upstream region from -651 to -109, without any promoter elements, CAAT-box, or TATA-box, could confer CO(2) and light responsiveness on the beta(2)-tubulin minimal promoter. This 543-bp region was divided into two parts: a 358-bp silencer region from -651 to -294, which represses the minimal promoter activity under high-CO(2) conditions, and a 185-bp enhancer region from -293 to -109, which activates the promoter under low-CO(2) conditions in the presence of light.

Carbonic anhydrase in eukaryotic algae: characterization, regulation, and possible function during photosynthesis

Carbonic anhydrase (CA) speeds up the equilibrium between CO2 and HCO3- at physiological pH values and has been detected in almost every species of the animal and plant kingdoms. Among eucaryotic micro- and macro-algae the enzyme is widely distributed and plays an important role in photosynthetic CO2 fixation. In some cases, different forms of carbonic anhydrases located extracellularly and intracellularly have been found to occur in the same cell. The expression of the genes encoding these CA isoforms are under the control of the inorganic carbon concentration in the medium, as the activities increase with decreasing the inorganic carbon content. Considerable progress has been made in recent years in isolating and characterizing the various forms of carbonic anhydrases on a biochemical and molecular level. Most of the data have been collected for microalgae like Chlamydomonas reinhardtii (Dangeard), while the situation in macroalgae is still descriptive. Therefore, this review summarizes the recent development with an emphasis on microalgae carbonic anhydrases.Key words: carbonic anhydrase, CO2 concentrating mechanism, macroalgae, microalgae, photosynthesis.

Induction and Regulation of Expression of a Low-CO2-Induced Mitochondrial Carbonic Anhydrase in Chlamydomonas reinhardtii

The time course of and the influence of light intensity and light quality on the induction of a mitochondrial carbonic anhydrase (CA) in the unicellular green alga Chlamydomonas reinhardtii was characterized using western and northern blots. This CA was expressed only under low-CO2 conditions (ambient air). In asynchronously grown cells, the mRNA was detected 15 min after transfer from air containing 5% CO2 to ambient air, and the 21-kD polypeptide was detected on western blots after 1 h. When transferred back to air containing 5% CO2, the mRNA disappeared within 1 h and the polypeptide was degraded within 3 d. Photosynthesis was required for the induction in asynchronous cultures. The induction increased with light up to 500 mumol m-2 s-1, where saturation occurred. In cells grown synchronously, however, expression of the mitochondrial CA was also detected in darkness. Under such conditions the expression followed a circadian rhythm, with mRNA appearing in the dark 30 min before the light was turned on. Algae left in darkness continued this rhythm for several days.