An update on carotenoid biosynthesis in algae: phylogenetic evidence for the existence of two classes of phytoene synthase

Deciphering the β-carotene hyperaccumulation in Dunaliella by the comprehensive analysis of Dunaliella salina and Dunaliella tertiolecta under high light conditions

The green microalga Dunaliella salina hyperaccumulates β-carotene in the chloroplast, which turns its cells orange. This does not occur in the sister species Dunaliella tertiolecta. However, the molecular mechanisms of β-carotene hyperaccumulation were still unclear. Here, we discovered the reasons for β-carotene hyperaccumulation by comparing the morphology, physiology, genome, and transcriptome between the carotenogenic D. salina and the noncarotenogenic D. tertiolecta after transfer to high light. The differences in photosynthetic capacity, cell growth, and the concentration of stored carbon suggest that these species regulate the supply and utilization of carbon differently. The number of β-carotene-containing plastid lipid globules increased in both species, but much faster and to a greater extent in D. salina than in D. tertiolecta. Consistent with the accumulation of plastid lipid globules, the expression of the methyl-erythritol-phosphate and carotenoid biosynthetic pathways increased only in D. salina, which explains the de novo synthesis of β-carotene. In D. salina, the concomitantly upregulated expression of the carotene globule proteins suggests that hyperaccumulation of β-carotene also requires a simultaneous increase in its sink capacity. Based on genomic analysis, we propose that D. salina has genetic advantages for routing carbon from growth to carotenoid metabolism.

Roles of Two Phytoene Synthases and Orange Protein in Carotenoid Metabolism of the β-Carotene-Accumulating Dunaliella salina

Phytoene synthase (PSY) is a key enzyme in carotenoid metabolism and often regulated by orange protein. However, few studies have focused on the functional differentiation of the two PSYs and their regulation by protein interaction in the β-carotene-accumulating Dunaliella salina CCAP 19/18. In this study, we confirmed that DsPSY1 from D. salina possessed high PSY catalytic activity, whereas DsPSY2 almost had no activity. Two amino acid residues at positions 144 and 285 responsible for substrate binding were associated with the functional variance between DsPSY1 and DsPSY2. Moreover, orange protein from D. salina (DsOR) could interact with DsPSY1/2. DbPSY from Dunaliella sp. FACHB-847 also had high PSY activity, but DbOR could not interact with DbPSY, which might be one reason why it could not highly accumulate β-carotene. Overexpression of DsOR, especially the mutant DsORHis, could significantly improve the single-cell carotenoid content and change cell morphology (with larger cell size, bigger plastoglobuli, and fragmented starch granules) of D. salina. Overall, DsPSY1 played a dominant role in carotenoid biosynthesis in D. salina, and DsOR promoted carotenoid accumulation, especially β-carotene via interacting with DsPSY1/2 and regulating the plastid development. Our study provides a new clue for the regulatory mechanism of carotenoid metabolism in Dunaliella. IMPORTANCE Phytoene synthase (PSY) as the key rate-limiting enzyme in carotenoid metabolism can be regulated by various regulators and factors. We found that DsPSY1 played a dominant role in carotenogenesis in the β-carotene-accumulating Dunaliella salina, and two amino acid residues critical in the substrate binding were associated with the functional variance between DsPSY1 and DsPSY2. Orange protein from D. salina (DsOR) can promote carotenoid accumulation via interacting with DsPSY1/2 and regulating the plastid development, which provides new insights into the molecular mechanism of massive accumulation of β-carotene in D. salina.

Technological advances in the production of carotenoids and their applications- A critical review

Carotenoids are naturally occurring pigments that are widely distributed in algae, fungi, bacteria, and plants. Carotenoids play a significant role in the food, feed, cosmetic, nutraceutical, and pharmaceutical industries. These pigments are effectively considered as a health-promoting compounds, which are widely used in our daily diet to reduce the risk of chronic diseases such as cardiovascular diseases, cancer, acute lung injury, cataracts, neural disorders, etc. In this context, this review paper demonstrates the synthesis of carotenoids and their potential application in the food and pharmaceutical industries. However, the demand for carotenoid production is increasing overtime, and the extraction and production are expensive and technically challenging. The recent developments in carotenoid biosynthesis, and key challenges, bottlenecks, and future perspectives were also discussed to enhance the circular bioeconomy.

Biotechnological production of β-carotene using plant in vitro cultures

β-carotene is biologically active compound widely distributed in plants. The use of plant in vitro cultures and genetic engineering is a promising strategy for its sustainable production. β-carotene is an orange carotenoid often found in leaves as well as in fruits, flowers, and roots. A member of the tetraterpene family, this 40-carbon isoprenoid has a conjugated double-bond structure, which is responsible for some of its most remarkable properties. In plants, β-carotene functions as an antenna pigment and antioxidant, providing protection against photooxidative damage caused by strong UV-B light. In humans, β-carotene acts as a precursor of vitamin A, prevents skin damage by solar radiation, and protects against several types of cancer such as oral, colon and prostate. Due to its wide spectrum of applications, the global market for β-carotene is expanding, and the demand can no longer be met by extraction from plant raw materials. Considerable research has been dedicated to finding more efficient production alternatives based on biotechnological systems. This review provides a detailed overview of the strategies used to increase the production of β-carotene in plant in vitro cultures, with particular focus on culture conditions, precursor feeding and elicitation, and the application of metabolic engineering.

Bioinformatics Analysis and Identification of Phytoene Synthase Gene in Microalgae

BACKGROUND

Carotenoids are known as lipophilic secondary metabolites with important biological activities, which are mostly used in the food and pharmaceutical industry. They contribute to the colours of many fruits and flowers. Studies on the biosynthetic pathways of isoprenoids and carotenoids are still scarce, especially in microalgae, and are limited to specific groups Dunaliella spp. In the Chlorophyta taxon of algae, the 2-C-methyl-D-erythritol 4-phosphate/1-deoxy-D-xylulose 5-phosphate (DOXP/MEP) is the synthesis pathway of sterols and carotenoids.

OBJECTIVES

In this study, we used 12 Psy gene sequences in Dunaliella sp., also Scenedesmus acutus, and Diospyros kaki to investigate a genome-wide search. The results are useful for better identification of carotenoids metabolisms and increasing the production rate of beta-carotene in pharmaceutical, food, and industrial processes.

METHODS

Phytoene synthase (Psy) from Dunaliella spp. was selected as the first regulatory point in the carotenoids pathway that catalysis the formation of geranylgeranyl pyrophosphate in isoprenoid biosynthesis. Structural, evolutionary, and physics-chemical characteristics were investigated using various bioinformatics tools and computer techniques. Moreover, some recently published patents were also regarded.

RESULTS

The maximum length of the conserved motif was 5167 bp for Dunaliella. sp. (DQ463306.1), and the smallest length of the conserved motif was 416 bp belong to D. salina (JQ762451.1). The average molecular weight of species was 41820.53 Da. The theoretical pI of species varied from 4.87 to 9.65, indicating vernation in the acidic nature. Two strains of D. bardawil (U91900.1 and EU328287.1) showed just a long-distance relationship with all other Dunaliella strains. Whilst, D. parva displayed the furthest vicinity with all the studied strains.

CONCLUSION

Our study highlighted the Psy regulatory mechanism as a key factor in the carotenoids pathway to facilitate genetic and metabolic engineering studies. The obtained three-dimensional arrangement of the amino acids revealed the regional structures and folding of the diverse segments of helices, sheets, turns. This information is a key point to unveil the protein's operation mechanism. Besides, we confirmed the suitability of bioinformatic approaches for analysing the gene structures and identifying the new Psy genes in unstudied microalgal strains.

A hypothesis about the origin of carotenoid lipid droplets in the green algae Dunaliella and Haematococcus

Hypercarotenogenesis in green algae evolved by mutation of PSY that increased its transcription at high light, disintegration of the eyespot in Dunaliella and acquisition of the capacity to export carotenoids from chloroplasts in Haematococcus. Carotenoids (Car) are lipid-soluble pigments synthesized in plants, algae, bacteria and fungi. Car have strong antioxidative properties and as such are utilized to reduce the danger of different diseases in humans. Two green microalgae are utilized as rich natural sources for Car: Dunaliella salina/bardawil accumulates 10% (w/w) β-carotene (βC), which is also pro-vitamin A, and Haematococcus pluvialis accumulates 4% (w/w) astaxanthin (Ast), the strongest antioxidant among Car. D. bardawil accumulates βC in plastoglobules within the chloroplast, whereas H. pluvialis deposits Ast in cytoplasmic lipid droplets (CLD). In this review we compare the hypercarotenogenic responses (HCR) in Dunaliella and in Haematococcus and try to outline hypothetical evolutionary pathways for its origin. We propose that a mutation in phytoene synthetase that increased its transcription level in response to high light stress had a pivotal role in the evolution of the HCR. Proteomic analyses indicated that in D. bardawil/salina the HCR evolved from dissociation and amplification of eyespot lipid globules. The more robust HCR in algae that accumulate carotenoids in CLD, such as H. pluvialis, required also acquisition of the capacity to export βC out of the chloroplast and its enzymatic conversion into Ast.

Differential gene expression for carotenoid biosynthesis in a green alga Ulva prolifera based on transcriptome analysis

BACKGROUND

Carotenoids are widely distributed in plants and algae, and their biosynthesis has attracted widespread interest. Carotenoid-related research has mostly focused on model species, and there is a lack of data on the carotenoid biosynthetic pathway in U. prolifera that is the main species leading to green tide, a harmful plague of floating green algae.

RESULTS

The carotenoid content of U. prolifera samples, that is the main species leading to green tide, a harmful plague of floating green algae at different temperatures revealed that its terpenoid was highest in the samples subjected to high temperature at 28 °C (H), followed by the samples subjected to low temperature at 12 °C (L). Its terpenoid was lowest in the samples subjected to medium temperature at 20 °C (M). We conducted transcriptome sequencing (148.5 million raw reads and 49,676 unigenes in total) of samples that were subjected to different temperatures to study the carotenoid biosynthesis of U. prolifera. There were 1125-3164 significant differentially expressed genes between L, M and H incubation temperatures, of which 11-672 genes were upregulated and 453-3102 genes were downregulated. A total of 3164 genes were significantly differentially expressed between H and M, of which 62 genes were upregulated and 3102 genes were downregulated. A total of 2669 significant differentially expressed genes were observed between L and H, of which 11 genes were upregulated and 2658 genes were downregulated. A total of 13 genes were identified to be involved in carotenoid biosynthesis in U. prolifera, and the expression levels of the majority were highest at H and lowest at M of incubation temperature. Both the carotenoid concentrations and the expression of the analysed genes were lowest in the normal temperature group, while low temperature and high temperature seemed to activate the biosynthesis of carotenoids in U. prolifera.

CONCLUSIONS

In this study, transcriptome sequencing provided critical information for understanding the accumulation of carotenoids and will serve as an important reference for the study of other metabolic pathways in U. prolifera.

Transcriptome sequencing and annotation of the halophytic microalga Dunaliella salina

The unicellular green alga Dunaliella salina is well adapted to salt stress and contains compounds (including β-carotene and vitamins) with potential commercial value. A large transcriptome database of D. salina during the adjustment, exponential and stationary growth phases was generated using a high throughput sequencing platform. We characterized the metabolic processes in D. salina with a focus on valuable metabolites, with the aim of manipulating D. salina to achieve greater economic value in large-scale production through a bioengineering strategy. Gene expression profiles under salt stress verified using quantitative polymerase chain reaction (qPCR) implied that salt can regulate the expression of key genes. This study generated a substantial fraction of D. salina transcriptional sequences for the entire growth cycle, providing a basis for the discovery of novel genes. This first full-scale transcriptome study of D. salina establishes a foundation for further comparative genomic studies.

Carotenoids biosynthesis and cleavage related genes from bacteria to plants

Carotenoids are essential for photosynthesis and photoprotection in photosynthetic organisms and beneficial for human health. Apocarotenoids derived from carotenoid degradation can serve critical functions including hormones, volatiles, and signals. They have been used commercially as food colorants, animal feed supplements, and nutraceuticals for cosmetic and pharmaceutical purposes. This review focuses on the molecular evolution of carotenogenic enzymes and carotenoid cleavage oxygenases (CCOs) from bacteria, fungi, cyanobacteria, algae, and plants. The diversity of carotenoids and apocarotenoids as well as their complicated biosynthetic pathway in different species can shed light on the history of early molecular evolution. Some carotenogenic genes (such as phytoene synthases) have high protein sequence similarity from bacteria to land plants, but some (such as phytoene desaturases, lycopene cyclases, carotenoid hydroxylases, and CCOs) have low similarity. The broad diversity of apocarotenoid volatile compounds can be attributed to large numbers of carotenoid precursors and the various cleavage sites catalyzed by CCOs enzymes. A variety of carotenogenic enzymes and CCOs indicate the functional diversification of carotenoids and apocrotenoids in different species. New carotenoids, new apocarotenoids, new carotenogenic enzymes, new CCOs, and new pathways still need to be explored.

Carotenoids in Microalgae

Carotenoids are a class of isoprenoids synthesized by all photosynthetic organisms as well as by some non-photosynthetic bacteria and fungi with broad applications in food, feed and cosmetics, and also in the nutraceutical and pharmaceutical industries. Microalgae represent an important source of high-value products, which include carotenoids, among others. Carotenoids play key roles in light harvesting and energy transfer during photosynthesis and in the protection of the photosynthetic apparatus against photooxidative damage. Carotenoids are generally divided into carotenes and xanthophyls, but accumulation in microalgae can also be classified as primary (essential for survival) and secondary (by exposure to specific stimuli).In this chapter, we outline the high value carotenoids produced by commercially important microalgae, their production pathways, the improved production rates that can be achieved by genetic engineering as well as their biotechnological applications.

Isolation and Analysis of the Cppsy Gene and Promoter from Chlorella protothecoides CS-41

Phytoene synthase (PSY) catalyzes the condensation of two molecules of geranylgeranyl pyrophosphate to form phytoene, the first colorless carotene in the carotenoid biosynthesis pathway. So it is regarded as the crucial enzyme for carotenoid production, and has unsurprisingly been involved in genetic engineering studies of carotenoid production. In this study, the psy gene from Chlorella protothecoides CS-41, designated Cppsy, was cloned using rapid amplification of cDNA ends. The full-length DNA was 2488 bp, and the corresponding cDNA was 1143 bp, which encoded 380 amino acids. Computational analysis suggested that this protein belongs to the Isoprenoid_Biosyn_C1 superfamily. It contained the consensus sequence, including three predicted substrate-Mg²⁺ binding sites. The Cppsy gene promoter was also cloned and characterized. Analysis revealed several candidate motifs for the promoter, which exhibited light- and methyl jasmonate (MeJA)-responsive characteristics, as well as some typical domains universally discovered in promoter sequences, such as the TATA-box and CAAT-box. Light- and MeJA treatment showed that the Cppsy expression level was significantly enhanced by light and MeJA. These results provide a basis for genetically modifying the carotenoid biosynthesis pathway in C. protothecoides.

Effect of an Introduced Phytoene Synthase Gene Expression on Carotenoid Biosynthesis in the Marine Diatom Phaeodactylum tricornutum

Carotenoids exert beneficial effects on human health through their excellent antioxidant activity. To increase carotenoid productivity in the marine Pennales Phaeodactylum tricornutum, we genetically engineered the phytoene synthase gene (psy) to improve expression because RNA-sequencing analysis has suggested that the expression level of psy is lower than other enzyme-encoding genes that are involved in the carotenoid biosynthetic pathway. We isolated psy from P. tricornutum, and this gene was fused with the enhanced green fluorescent protein gene to detect psy expression. After transformation using the microparticle bombardment technique, we obtained several P. tricornutum transformants and confirmed psy expression in their plastids. We investigated the amounts of PSY mRNA and carotenoids, such as fucoxanthin and β-carotene, at different growth phases. The introduction of psy increased the fucoxanthin content of a transformants by approximately 1.45-fold relative to the levels in the wild-type diatom. However, some transformants failed to show a significant increase in the carotenoid content relative to that of the wild-type diatom. We also found that the amount of PSY mRNA at log phase might contribute to the increase in carotenoids in the transformants at stationary phase.

Hypoosmotic expression of Dunaliella bardawil ζ-carotene desaturase is attributed to a hypoosmolarity-responsive element different from other key carotenogenic genes

Some key carotenogenic genes (crts) in Dunaliella bardawil are regulated in response to salt stress partly due to salt-inducible cis-acting elements in their promoters. Thus, we isolated and compared the ζ-carotene desaturase (Dbzds) promoter with other crts promoters including phytoene synthase (Dbpsy), phytoene desaturase (Dbpds), and lycopene β-cyclase1 (DblycB1) to identify salt-inducible element(s) in the Dbzds promoter. In silico analysis of the Dbzds promoter found several potential cis-acting elements, such as abscisic acid response element-like sequence, myelocytomatosis oncogene1 recognition motif, AGC box, anaerobic motif2, and activation sequence factor1 binding site. Remarkably, instead of salt-inducible elements, we found a unique regulatory sequence architecture in the Dbzds promoter: a hypoosmolarity-responsive element (HRE) candidate followed by a potential hypoosmolarity-inducible factor GBF5 binding site. Deletion experiments demonstrated that only HRE, but not the GBF5 binding site, is responsible for hypoosmotic expression of the fusion of Zeocin resistance gene (ble) to the enhanced green fluorescent protein (egfp) chimeric gene under salt stress. Dbzds transcripts were in accordance with those of ble-egfp driven by the wild-type Dbzds promoter. Consequently, Dbzds is hypoosmotically regulated by its promoter, and HRE is responsible for this hypoosmotic response. Finally, the hypoosmolarity mechanism of Dbzds was studied by comparing transcript profiles and regulatory elements of Dbzds with those of Dbpsy, Dbpds, DblycB1, and DblycB2, revealing that different induction characteristics of crts may correlate with regulatory sequence architecture.

Pathways of carotenoid biosynthesis in bacteria and microalgae

The carotenoids, a subfamily of the isoprenoids, are among the most widespread, ancient, diverse, and rich class of all natural products and biomolecules. Microorganisms, as well as microalgae and bacteria synthesize isoprenoids from isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). For long time, mevalonic acid was assumed to be the only natural precursor for IPP and DMAPP in the cytosolic acetate/mevalonate pathway for the biosynthesis of sterols, sesquiterpenes, triterpenoids, and carotenoids. At present, it is accepted that the relatively new route, the methylerythritol 4-phosphate (MEP), or 1-deoxy-D: -xylulose-5-phosphate (DOXP) is the main pathway for the biosynthesis of plastidic isoprenoids, such as carotenoids, phytol (a side chain of chlorophylls), plastoquinone-9, isoprene, mono-, and diterpenes. Cytosolic isoprenoids (sterols) biosynthesized by MEP have been reported in eubacteria and algae (Chlorella, Chlamydomonas, Scenedesmus, and Dunaliella). This review summarizes current knowledge of the biosynthetic pathways leading to the formation of different isoprenoids and carotenoids in bacteria and microalgae. Particular attention was given to the last early steps of the biosynthesis of the key C(5)-precursor and the final steps of the biosynthesis of carotenoids including selected examples in microalgae and bacteria as well as the recent advances in genomics and metabolic engineering.

Expression, subcellular localization, and cis-regulatory structure of duplicated phytoene synthase genes in melon (Cucumis melo L.)

Carotenoids perform many critical functions in plants, animals, and humans. It is therefore important to understand carotenoid biosynthesis and its regulation in plants. Phytoene synthase (PSY) catalyzes the first committed and rate-limiting step in carotenoid biosynthesis. While PSY is present as a single copy gene in Arabidopsis, duplicated PSY genes have been identified in many economically important monocot and dicot crops. CmPSY1 was previously identified from melon (Cucumis melo L.), but was not functionally characterized. We isolated a second PSY gene, CmPSY2, from melon in this work. CmPSY2 possesses a unique intron/exon structure that has not been observed in other plant PSYs. Both CmPSY1 and CmPSY2 are functional in vitro, but exhibit distinct expression patterns in different melon tissues and during fruit development, suggesting differential regulation of the duplicated melon PSY genes. In vitro chloroplast import assays verified the plastidic localization of CmPSY1 and CmPSY2 despite the lack of an obvious plastid target peptide in CmPSY2. Promoter motif analysis of the duplicated melon and tomato PSY genes and the Arabidopsis PSY revealed distinctive cis-regulatory structures of melon PSYs and identified gibberellin-responsive motifs in all PSYs except for SlPSY1, which has not been reported previously. Overall, these data provide new insights into the evolutionary history of plant PSY genes and the regulation of PSY expression by developmental and environmental signals that may involve different regulatory networks.

Do red and green make brown?: perspectives on plastid acquisitions within chromalveolates

The chromalveolate "supergroup" is of key interest in contemporary phycology, as it contains the overwhelming majority of extant algal species, including several phyla of key importance to oceanic net primary productivity such as diatoms, kelps, and dinoflagellates. There is also intense current interest in the exploitation of these algae for industrial purposes, such as biodiesel production. However, the evolution of the constituent species, and in particular the origin and radiation of the chloroplast genomes, remains poorly understood. In this review, we discuss current theories of the origins of the extant red alga-derived chloroplast lineages in the chromalveolates and the potential ramifications of the recent discovery of large numbers of green algal genes in chromalveolate genomes. We consider that the best explanation for this is that chromalveolates historically possessed a cryptic green algal endosymbiont that was subsequently replaced by a red algal chloroplast. We consider how changing selective pressures acting on ancient chromalveolate lineages may have selectively favored the serial endosymbioses of green and red algae and whether a complex endosymbiotic history facilitated the rise of chromalveolates to their current position of ecological prominence.

Enhancement of carotenoids biosynthesis in Chlamydomonas reinhardtii by nuclear transformation using a phytoene synthase gene isolated from Chlorella zofingiensis

The isolation and characterization of the phytoene synthase gene from the green microalga Chlorella zofingiensis (CzPSY), involved in the first step of the carotenoids biosynthetic pathway, have been performed. CzPSY gene encodes a polypeptide of 420 amino acids. A single copy of CzPSY has been found in C. zofingiensis by Southern blot analysis. Heterologous genetic complementation in Escherichia coli showed the ability of the predicted protein to catalyze the condensation of two molecules of geranylgeranyl pyrophosphate (GGPP) to form phytoene. Phylogenetic analysis has shown that the deduced protein forms a cluster with the rest of the phytoene synthases (PSY) of the chlorophycean microalgae studied, being very closely related to PSY of plants. This new isolated gene has been adequately inserted in a vector and expressed in Chlamydomonas reinhardtii. The overexpression of CzPSY in C. reinhardtii, by nuclear transformation, has led to an increase in the corresponding CzPSY transcript level as well as in the content of the carotenoids violaxanthin and lutein which were 2.0- and 2.2-fold higher than in untransformed cells. This is an example of manipulation of the carotenogenic pathway in eukaryotic microalgae, which can open up the possibility of enhancing the productivity of commercial carotenoids by molecular engineering.

In silico analysis of phytoene synthase and its promoter reveals hints for regulation mechanisms of carotenogenesis in Duanliella bardawil

MOTIVATION

Previous researches showed that phytoene synthase (Psy) from Dunaliella bardawil is the first regulatory point in carotenogenesis. We hypothesize certain interactions between the environmental stress factors and the regulatory sequences of Psy in D.bardawil (DbPsy). Consequently, LA PCR-based genomic walking approach was performed for isolation of psy promoter and terminator, respectively. The obtained nucleic acid sequences and the corresponding protein structure of DbPsy were analyzed and predicted using various bioinformatics tools. Finally, we presented some hints for the regulation mechanisms of DbPsy at the molecular level according to the computed results.

RESULTS

LA PCR-based genomic walking results showed that the isolated sequences are the promoter and terminator of psy, correspondingly. Computational analysis demonstrated several candidate motifs of the promoter exhibiting hypothetic UV-B-, norglurzon- and salt-induced characteristics, as well as some typical domains universally discovered in promoter sequences, such as TATA-box, CCAAT-box and GATA-box, etc. Furthermore, the structure of Psy was also predicted and aligned along with many counterparts at the protein level. Low homology of N-terminus was found in D.bardawil, while a relatively conserved C-terminus was predicted to be involved in the catalytic activity and substrate recognization/binding. Phylogenic analysis classified the DbPsy into a cluster with other algae. These results implied that Psy may share similar regulation mechanisms among algae with respect to their C-termini; while the diversity in N-terminus among Psys, along with the predicted inducible motifs in psy promoter from D.bardawil, may confer the fine tuning differences between D.bardawil and other algae.

CONCLUSION

By means of computer techniques, we found in D.barawali that two interesting conserved motifs of psy promoter may involve in UV-B, norglurzon and salt regulation correspondingly; and that the diversity of Psy protein mainly lies in the N-termini among algae. These results indicate some hints for regulation mechanisms of carotenogenesis in D.bradawil.

CONTACT

jgjiang@scut.edu.cn.

Carotenoids and their cleavage products: biosynthesis and functions

This review focuses on plant carotenoids, but it also includes progress made on microbial and animal carotenoid metabolism to better understand the functions and the evolution of these structurally diverse compounds with a common backbone. Plants have evolved isogenes for specific key steps of carotenoid biosynthesis with differential expression profiles, whose characteristic features will be compared. Perhaps the most exciting progress has been made in studies of carotenoid cleavage products (apocarotenoids) with an ever-expanding variety of novel functions being discovered. This review therefore covers structural, molecular genetic and functional aspects of carotenoids and apocarotenoids alike. Apocarotenoids are specifically tailored from carotenoids by a family of oxidative cleavage enzymes, but whether there are contributions to their generation from chemical oxidation, photooxidation or other mechanisms is largely unknown. Control of carotenoid homeostasis is discussed in the context of biosynthetic and degradative reactions but also in the context of subcellular environments for deposition and sequestration within and outside of plastids. Other aspects of carotenoid research, including metabolic engineering and synthetic biology approaches, will only be covered briefly.

Expressed sequence tag (EST) profiling in hyper saline shocked Dunaliella salina reveals high expression of protein synthetic apparatus components

The unicellular, halotolerant, green alga, Dunaliella salina (Chlorophyceae) has the unique ability to adapt and grow in a wide range of salt conditions from about 0.05 to 5.5M. To better understand the molecular basis of its salinity tolerance, a complementary DNA (cDNA) library was constructed from D. salina cells adapted to 2.5M NaCl, salt-shocked at 3.4M NaCl for 5h, and used to generate an expressed sequence tag (EST) database. ESTs were obtained for 2831 clones representing 1401 unique transcripts. Putative functions were assigned to 1901 (67.2%) ESTs after comparison with protein databases. An additional 154 (5.4%) ESTs had significant similarity to known sequences whose functions are unclear and 776 (27.4%) had no similarity to known sequences. For those D. salina ESTs for which functional assignments could be made, the largest functional categories included protein synthesis (35.7%), energy (photosynthesis) (21.4%), primary metabolism (13.8%) and protein fate (6.8%). Within the protein synthesis category, the vast majority of ESTs (80.3%) encoded ribosomal proteins representing about 95% of the approximately 82 subunits of the cytosolic ribosome indicating that D. salina invests substantial resources in the production and maintenance of protein synthesis. The increased mRNA expression upon salinity shock was verified for a small set of selected genes by real-time, quantitative reverse-transcription-polymerase chain reaction (qRT-PCR). This EST collection also provided important new insights into the genetic underpinnings for the biosynthesis and utilization of glycerol and other osmoprotectants, the carotenoid biosynthetic pathway, reactive oxygen-scavenging enzymes, and molecular chaperones (heat shock proteins) not described previously for D. salina. EST discovery also revealed the existence of RNA interference and signaling pathways associated with osmotic stress adaptation. The unknown ESTs described here provide a rich resource for the identification of novel genes associated with the mechanistic basis of salinity stress tolerance and other stress-adaptive traits.

Carotenoid biosynthesis in diatoms

Diatoms are ubiquitous and constitute an important group of the phytoplankton community having a major contribution to the total marine primary production. These microalgae exhibit a characteristic golden-brown colour due to a high amount of the xanthophyll fucoxanthin that plays a major role in the light-harvesting complex of photosystems. In the water column, diatoms are exposed to light intensities that vary quickly from lower to higher values. Xanthophyll cycles prevent photodestruction of the cells in excessive light intensities. In diatoms, the diadinoxanthin-diatoxanthin cycle is the most important short-term photoprotective mechanism. If the biosynthetic pathways of chloroplast pigments have been extensively studied in higher plants and green algae, the research on carotenoid biosynthesis in diatoms is still in its infancy. In this study, the data on the biosynthetic pathway of diatom carotenoids are reviewed. The early steps occur through the 2-C-methyl-D: -erythritol 4-phosphate (MEP) pathway. Then a hypothetical pathway is suggested from dimethylallyl diphosphate (DMAPP) and isopentenyl pyrophosphate (IPP). Most of the enzymes of the pathway have not been so far isolated from diatoms, but candidate genes for each of them were identified using protein similarity searches of genomic data.

Coordinated regulation of gene expression for carotenoid metabolism in Chlamydomonas reinhardtii

Carotenoids are important plant pigments for both light harvesting and photooxidation protection. Using the model system of the unicellular green alga Chlamydomonas reinhardtii, we characterized the regulation of gene expression for carotenoid metabolism by quantifying changes in the transcript abundance of dxs, dxr and ipi in the plastidic methylerythritol phosphate pathway and of ggps, psy, pds, lcyb and bchy, directly involved in carotenoid metabolism, under different photoperiod, light and metabolite treatments. The expression of these genes fluctuated with light/dark shifting. Light treatment also promoted the accumulation of transcripts of all these genes. Of the genes studied, dxs, ggps and lcyb displayed the typical circadian pattern by retaining a rhythmic fluctuation of transcript abundance under both constant light and constant dark entrainments. The expression of these genes could also be regulated by metabolic intermediates. For example, ggps was significantly suppressed by a geranylgeranyl pyrophosphate supplement and ipi was upregulated by isopentenyl pyrophosphate. Furthermore, CrOr, a C. reinhardtii homolog of the recently characterized Or gene that accounts for carotenoid accumulation, also showed co-expression with carotenoid biosynthetic genes such as pds and lcyb. Our data suggest a coordinated regulation on carotenoid metabolism in C. reinhardtii at the transcriptional level.