Red Alga Galdieria Sulphuraria Research Articles

Profiling of lipid and glycogen accumulations under different growth conditions in the sulfothermophilic red alga Galdieria sulphuraria

The unicellular red alga Galdieria sulphuraria grows efficiently and produces a large amount of biomass in acidic conditions at high temperatures. It has great potential to produce biofuels and other beneficial compounds without becoming contaminated with other organisms. In G. sulphuraria, biomass measurements and glycogen and lipid analyses demonstrated that the amounts and compositions of glycogen and lipids differed when cells were grown under autotrophic, mixotrophic, and heterotrophic conditions. Maximum biomass production was obtained in the mixotrophic culture. High amounts of glycogen were obtained in the mixotrophic cultures, while the amounts of neutral lipids were similar between mixotrophic and heterotrophic cultures. The amounts of neutral lipids were highest in red algae, including thermophiles. Glycogen structure and fatty acids compositions largely depended on the growth conditions.

Floridoside and isofloridoside are synthesized by trehalose 6-phosphate synthase-like enzymes in the red alga Galdieria sulphuraria.

Compatible solutes are small molecules that are involved in acclimation to various abiotic stresses, especially high salinity. Among the red algae, the main photosynthetic products floridoside and isofloridoside (galactosylglycerols) are known also to contribute to the osmotic acclimation of cells. However, the genes encoding (iso)floridoside biosynthetic enzymes are still unknown. To identify candidate genes, we examined the genome of the floridoside- and isofloridoside-accumulating extremophilic red alga Galdieria sulphuraria belonging to the Cyanidiales. We hypothesized that two candidate genes, Gasu_10960 and Gasu_26940, code for enzymes involved in floridoside and isofloridoside biosynthesis. These proteins comprise a sugar phosphate synthase and a sugar phosphate phosphatase domain. To verify their biochemical activity, both genes were in vitro translated into the entire proteins. The protein translation mixture containing Gasu_10960 synthesized small amounts of isofloridoside, whereas the Gasu_26940 translation mix also produced small amounts of floridoside. Moreover, the expression of Gasu_10960 in a salt-sensitive mutant of the cyanobacterium Synechocystis sp. PCC 6803 resulted in increased salt tolerance as a consequence of the presence of isofloridoside in the complemented cells. Thus, our experiments suggest that the Gasu_26940 and Gasu_10960 genes of G.sulphuraria encode the enzymatically active floridoside and isofloridoside phosphate synthase/phosphatase fusion proteins, respectively, crucial for salt acclimation.

Isolation and characterization of thermostable phycocyanin from Galdieria sulphuraria

Phycocyanin is a highly valuable pigmented protein synthesized by several species of cyanobacteria and red alga. In this study we demonstrate the production of thermostable phycocyanin from the unicellular red alga Galdieria sulphuraria. Phycocyanin was extracted by repeated freeze-thaw cycles and purified in a two-step process using ammonium sulfate fractionation, at 25% and 50% concentrations. Purified phycocyanin exhibited maximum absorbance at 620 nm, and the purity ratio (A620/A280) was found to be greater than 4. The recovery efficiency of phycocyanin from the crude extract was above 80%. In total, approximately 19 milligram pure phycocyanin was obtained from 3 g of wet cell mass of Galdieria sp. Subunits α and β of the protein were separated by SDS-PAGE and analyzed by MALDITOF mass spectrometry for identification, which confirmed that the isolated protein is phycocyanin. The molecular weight of α and β subunits of phycocyanin was found to be 17.6 and 18.4 kDa, respectively.

Horizontal gene acquisitions by eukaryotes as drivers of adaptive evolution

In contrast to vertical gene transfer from parent to offspring, horizontal (or lateral) gene transfer moves genetic information between different species. Bacteria and archaea often adapt through horizontal gene transfer. Recent analyses indicate that eukaryotic genomes, too, have acquired numerous genes via horizontal transfer from prokaryotes and other lineages. Based on this we raise the hypothesis that horizontally acquired genes may have contributed more to adaptive evolution of eukaryotes than previously assumed. Current candidate sets of horizontally acquired eukaryotic genes may just be the tip of an iceberg. We have recently shown that adaptation of the thermoacidophilic red alga Galdieria sulphuraria to its hot, acid, toxic-metal laden, volcanic environment was facilitated by the acquisition of numerous genes from extremophile bacteria and archaea. Other recently published examples of horizontal acquisitions involved in adaptation include ice-binding proteins in marine algae, enzymes for carotenoid biosynthesis in aphids, and genes involved in fungal metabolism. Editor's suggested further reading in BioEssays Jumping the fine LINE between species: Horizontal transfer of transposable elements in animals catalyses genome evolutionAbstract.

Efficient Light Harvesting in a Dark, Hot, Acidic Environment: The Structure and Function of PSI-LHCI from Galdieria sulphuraria

Photosystem I-light harvesting complex I (PSI-LHCI) was isolated from the thermoacidophilic red alga Galdieria sulphuraria, and its structure, composition, and light-harvesting function were characterized by electron microscopy, mass spectrometry, and ultrafast optical spectroscopy. The results show that Galdieria PSI is a monomer with core features similar to those of PSI from green algae, but with significant differences in shape and size. A comparison with the crystal structure of higher plant (pea) PSI-LHCI indicates that Galdieria PSI binds seven to nine light-harvesting proteins. Results from ultrafast optical spectroscopy show that the functional coupling of the LHCI proteins to the PSI core is tighter than in other eukaryotic PSI-LHCI systems reported thus far. This tight coupling helps Galdieria perform efficient light harvesting under the low-light conditions present in its natural endolithic habitat.

Genetic analysis of the Photosystem I subunits from the red alga, Galdieria sulphuraria

Currently, there are very little data available regarding the photosynthetic apparatus of red algae. We have analyzed the genes for Photosystem I in the recently sequenced genome of the red alga Galdieria sulphuraria. All subunits that are conserved between plants and cyanobacteria were unambiguously identified in the Galdieria genome: PsaA, PsaB, PsaC, PsaD, PsaE, PsaF, PsaI, PsaJ, PsaK and PsaL. From the plant specific subunits, PsaN and PsaO were identified but the sequence homology was much lower than for the subunits that are present in plants and cyanobacteria. The subunit PsaX, which is specific for thermophilic cyanobacteria, is not present in the Galdieria genome, whereas PsaM is a plastid-encoded protein as in other red algae. The sequences of the core subunits of PSI were further analyzed by mapping of the conserved areas in the crystal structures of cyanobacterial and plant PSI. The structural comparison shows that PSI from the red alga Galdieria may represent a common ancestral structure at the interface between cyanobacterial and plant PSI. Some subunits have a “zwitter” structure that contains structural elements that show similarities with either plant or cyanobacterial PSI. The structure of PsaL, which is responsible for the trimerization of PSI in cyanobacteria, lacks a short helix and the Ca 2+ binding site, which are essential for trimer formation indicating that the Galdieria PSI is a monomer. However the sequence homology to plant PsaL is low and lacks strong conservation of the interaction sites with PsaH. Furthermore, the sites for interaction of plant PSI with the LHCI complex are not well conserved between plants and Galdieria, which may indicate that Galdieria may contain a PSI that is evolutionarily much more ancient than PSI from green algae, plants and the current cyanobacteria.

The genome of the thermoacidophilic red microalga Galdieria sulphuraria encodes a small family of secreted class III peroxidases that might be involved in cell wall modification

We report the identification of a small family of secreted class III plant peroxidases (Prx) from the genome of the unicellular thermoacidophilic red alga Galdieria sulphuraria (Cyanidiaceae). Apart from two class I ascorbate peroxidases and one cytochrome c peroxidase, the red algal genome encodes four class III plant peroxidases, thus complementing the short list of algal cell wall peroxidases (Passardi et al. in Genomics 89:567-579, 2007). We have characterized the family gene structure, analyzed the extracellular space and cell wall fraction of G. sulphuraria for the presence of peroxidase activity and used shotgun proteomics to identify candidate extracellular peroxidases. For a detailed enzymatic characterization, we have purified a secreted peroxidase (GsPrx04) from the cell-free medium using hydrophobic interaction chromatography. The enzyme proved heat and acid-stable and exhibited an apparent molecular mass of 40 kDa. Comparative genomics between endolithically growing G. sulphuraria and a close relative, the obligatory aquatic, cell wall-less Cyanidioschyzon merolae, revealed that class III peroxidases only occur in the terrestrial microalga, thus supporting the key function of these enzymes in the process of land colonization.

Structurally reduced monosaccharide transporters in an evolutionarily conserved red alga.

The unicellular red alga Galdieria sulphuraria is a facultative heterotrophic member of the Cyanidiaceae, a group of evolutionary highly conserved extremophilic red algae. Uptake of various sugars and polyols is accomplished by a large number of distinct plasma membrane transporters. We have cloned three transporters [GsSPT1 (G. sulphuraria sugar and polyol transporter 1), GsSPT2 and GsSPT4], followed their transcriptional regulation and assayed their transport capacities in the heterologous yeast system. SPT1 is a conserved type of sugar/H(+) symporter with 12 predicted transmembrane-spanning domains, whereas SPT2 and SPT4 represent monosaccharide transporters, characterized by only nine hydrophobic domains. Surprisingly, all three proteins are functional plasma membrane transporters, as demonstrated by genetic complementation of a sugar uptake-deficient yeast mutant. Substrate specificities were broad and largely redundant, except for glucose, which was only taken up by SPT1. Comparison of SPT1 and truncated SPT1(Delta1-3) indicated that the N-terminus of the protein is not required for sugar transport or substrate recognition. However, its deletion affected substrate affinity as well as maximal transport velocity and released the pH dependency of sugar uptake. In line with these results, uptake by SPT2 and SPT4 was active but not pH-dependent, making a H(+) symport mechanism unlikely for the truncated proteins. We postulate SPT2 and SPT4 as functional plasma membrane transporters in G. sulphuraria. Most likely, they originated from genes encoding active monosaccharide/H(+) symporters with 12 transmembrane-spanning domains.

Ultrastructure and enzyme complement of proplastids from heterotrophically grown cells of the red alga Galdieria sulphuraria

The unicellular thermoacidophilic red alga Galdieria sulphuraria is known to grow autotrophically as well as heterotrophically on a wide variety of organic carbon sources. Strain 107.79 becomes nearly colourless when grown heterotrophically on lactose. Under autotrophic conditions, a dominant chloroplast occupies most of the cell volume, while very small, yellow to colourless plastid-like organelles were observed in heterotrophic cells. Their exact number could not be established with certainty. To investigate the role of these algal proplastids, we developed a method to isolate intact organelles from heterotrophic cells via sucrose density-gradient centrifugation. Using electron microscopy, these organelles were shown to have a morphology similar to proplastids with a double membrane envelope, very few thylakoids and one to several dense bodies, apparently plastoglobuli. Investigation of the enzymatic capacities of isolated proplastids showed activity of triose phosphate isomerase and NAD-glyceraldehyde 3-phosphate dehydrogenase (GAPDH), but NADP-GAPDH, 3-phosphoglycerate kinase, fructose-1,6 P2 aldolase, fructose-1,6 P2 phosphatase, and malate dehydrogenase were not detected. These findings were supported by the pattern of plastid- and cytosol-specific isozymes of sugar phosphate metabolism and indicate that heterotrophically grown cells of G. sulphuraria 107.79 have not only lost their photosynthetic apparatus but also exhibit an extremely reduced enzyme complement in their dedifferentiated plastid organelle.

Redox Regulation of Chloroplast Enzymes in Galdieria sulphuraria in View of Eukaryotic Evolution

Redox modulation is a general mechanism for enzyme regulation, particularly for the post-translational regulation of the Calvin cycle in chloroplasts of green plants. Although red algae and photosynthetic protists that harbor plastids of red algal origin contribute greatly to global carbon fixation, relatively little is known about post-translational regulation of chloroplast enzymes in this important group of photosynthetic eukaryotes. To address this question, we used biochemistry, phylogenetics and analysis of recently completed genome sequences. We studied the functionality of the chloroplast enzymes phosphoribulokinase (PRK, EC 2.7.1.19), NADP-dependent glyceraldehyde 3-phosphate dehydrogenase (NADP-GAPDH, GapA, EC 1.2.1.13), fructose 1,6-bisphosphatase (FBPase, EC 3.1.3.11) and glucose 6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49), as well as NADP-malate dehydrogenase (NADP-MDH, EC 1.1.1.37) in the unicellular red alga Galdieria sulphuraria (Galdieri) Merola. Despite high sequence similarity of G. sulphuraria proteins to those of other photosynthetic organisms, we found a number of distinct differences. Both PRK and GAPDH co-eluted with CP12 in a high molecular weight complex in the presence of oxidized glutathione, although Galdieria CP12 lacks the two cysteines essential for the formation of the N-terminal peptide loop present in higher plants. However, PRK inactivation upon complex formation turned out to be incomplete. G6PDH was redox modulated, but remained in its tetrameric form; FBPase was poorly redox regulated, despite conservation of the two redox-active cysteines. No indication for the presence of plastidic NADP-MDH (and other components of the malate valve) was found.

Regulation of photosynthesis in the unicellular acidophilic red alga Galdieria sulphuraria†

Extremophilic organisms are gaining increasing interest because of their unique metabolic capacities and great biotechnological potential. The unicellular acidophilic and mesothermophilic red alga Galdieria sulphuraria (074G) can grow autotrophically in light as well as heterotrophically in the dark. In this paper, the effects of externally added glucose on primary and secondary photosynthetic reactions are assessed to elucidate mixotrophic capacities of the alga. Photosynthetic O2 evolution was quantified in an open system with a constant supply of CO2 to avoid rapid volatilization of dissolved inorganic carbon at low pH levels. In the presence of glucose, O2 evolution was repressed even in illuminated cells. Ratios of variable to maximum chlorophyll fluorescence (Fv/Fm) and 77 K fluorescence spectra indicated a reduced photochemical efficiency of photosystem II. The results were corroborated by strongly reduced levels of the photosystem II reaction centre protein D1. The downregulation of primary photosynthetic reactions was accompanied by reduced levels of the Calvin Cycle enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Both effects depended on functional sugar uptake and are thus initiated by intracellular rather than extracellular glucose. Following glucose depletion, photosynthetic O2 evolution of illuminated cells commenced after 15 h and Rubisco levels again reached the levels of autotrophic cells. It is concluded that true mixotrophy, involving electron transport across both photosystems, does not occur in G. sulphuraria 074G, and that heterotrophic growth is favoured over autotrophic growth if sufficient organic carbon is available.

Accumulation of phycocyanin in heterotrophic and mixotrophic cultures of the acidophilic red alga Galdieria sulphuraria

The relationship between light intensity, nitrogen availability and pigmentation was investigated in mixotrophic and heterotrophic cultures of the unicellular red alga Galdieria sulphuraria 074G, a potential host for production of the blue pigment, phycocyanin (PC). During the exponential growth phase of batch cultures, G. sulphuraria 074G contained 2–4 mg phycocyanin per g dry weight. In carbon-limited and nitrogen-sufficient batch cultures grown in darkness, this value increased to 8–12 mg g −1 dry weight during the stationary phase, whereas the phycocyanin content in nitrogen-deficient cells decreased to values below 1 mg g −1 dry weight during stationary phase. Light intensities between 0 and 100 μmol photons m −2 s −1 had no influence on phycocyanin accumulation in mixotrophic cultures grown on glucose or fructose, while light stimulated phycocyanin synthesis in cultures grown on glycerol, in which the phycocyanin content in stationary phase was increased from 10 mg g −1 dry weight in darkness to 20 mg g −1 dry weight at a light intensity of 80 μmol photons m −2 s −1. At higher light intensities, less phycocyanin accumulated than at lower intensities, irrespective of the carbon substrate used. In carbon-limited continuous flow cultures grown on glucose or glycerol at a dilution rate of 0.63 day −1, corresponding to 50% of the maximum specific growth rate, the highest steady-state phycocyanin content of 15–28 mg g −1 dry weight was found at 65 μmol photons m −2 s −1. In contrast to the apparent glucose repression of light-induced PC synthesis observed in batch cultures, no glucose repression of the light stimulation was observed in continuous flow cultures because the glucose concentration in the culture supernatant always remained at limiting levels. Despite the fact that G. sulphuraria 074G contains less phycocyanin than some other microalgae and cyanobacteria, the ability of G. sulphuraria 074G to grow and synthesize phycocyanin in heterotrophic or mixotrophic cultures makes it an interesting alternative to the cyanobacterium, Spirulina platensis presently used for synthesis of phycocyanin.

Comparative Genomics of Two Closely Related Unicellular Thermo-Acidophilic Red Algae, Galdieria sulphuraria and Cyanidioschyzon merolae, Reveals the Molecular Basis of the Metabolic Flexibility of Galdieria sulphuraria and Significant Differences in Carbohydrate Metabolism of Both Algae

Unicellular algae serve as models for the study and discovery of metabolic pathways, for the functional dissection of cell biological processes such as organellar division and cell motility, and for the identification of novel genes and gene functions. The recent completion of several algal genome sequences and expressed sequence tag collections and the establishment of nuclear and organellar transformation methods has opened the way for functional genomics approaches using algal model systems. The thermo-acidophilic unicellular red alga Galdieria sulphuraria represents a particularly interesting species for a genomics approach owing to its extraordinary metabolic versatility such as heterotrophic and mixotrophic growth on more than 50 different carbon sources and its adaptation to hot acidic environments. However, the ab initio prediction of genes required for unknown metabolic pathways from genome sequences is not trivial. A compelling strategy for gene identification is the comparison of similarly sized genomes of related organisms with different physiologies. Using this approach, candidate genes were identified that are critical to the metabolic versatility of Galdieria. Expressed sequence tags and high-throughput genomic sequence reads covering >70% of the G. sulphuraria genome were compared to the genome of the unicellular, obligate photoautotrophic red alga Cyanidioschyzon merolae. More than 30% of the Galdieria sequences did not relate to any of the Cyanidioschyzon genes. A closer inspection of these sequences revealed a large number of membrane transporters and enzymes of carbohydrate metabolism that are unique to Galdieria. Based on these data, it is proposed that genes involved in the uptake of reduced carbon compounds and enzymes involved in their metabolism are crucial to the metabolic flexibility of G. sulphuraria.

Chloroplast fructose 1,6-bisphosphatase with changed redox modulation: comparison of the Galdieria enzyme with cysteine mutants from spinach

Spinach fructose 1,6-bisphosphatase (FBPase, EC 3.1.3.11), a redox-modulated chloroplast enzyme and part of the Calvin cycle, and three different Cys mutants were expressed in E. coli. The properties of the purified proteins were compared to those of native and recombinant chloroplast FBPase from the red alga Galdieria sulphuraria. In spinach chloroplast FBPase, Cys 155 and Cys 174 are engaged in the formation of the disulfide bridge. The corresponding mutants are active when expressed in E. coli, while C179S is inactive and can be reductively activated as can the wild-type enzyme. The active C174S mutant, however, could be inactivated by oxidation, and reactivated, but only by reduction, not alternatively with high pH and high Mg 2+ as is the case for the wild-type enzyme. In the sequence of Galdieria FBPase, the Cys that corresponds to Cys 179 in the spinach enzyme is lacking. However, the Galdieria FBPase, in contrast to the spinach Cys 179 mutant, does not show any indication for a comparable redox modulation of its activity. Instead, oxidation only leads to partial inactivation without any qualitative changes in enzyme properties. Upon reduction, the lost activity can be recovered.

Hot sulphur springs at São Miguel Island (Azores) : A new locality of the acidophilic red alga Galdieria sulphuraria (Galdieri) Merola

All three highly acidic volcanic areas examined on the Azorian island of Sao Miguel were inhabited by the red alga Galdieria sulphuraria (GALDIERI) MEROLA. The morphology and physiology of the isolates from Sao Miguel did not differ significantly from isolates collected in other parts of the world. Similar to other isolates the maximum growth temperature was about 56 °C, and growth was optimal at pH 1-2. Growth rates under auto- and heterotrophic conditions were comparable. Especially at a site in the town of Furnas extensive cryptoendolithic growth of G. sulphuraria was observed. Means of dispersal are discussed for this extremophilic alga.

Different Sugar Kinases Are Involved in the Sugar Sensing ofGaldieria sulphuraria

The unicellular acidophilic red alga Galdieria sulphuraria is a facultative heterotroph with a complex uptake system for sugars and polyols, consisting of at least 14 transporters. Upon transfer to heterotrophic conditions, these transporters were induced simultaneously. Once induced, transporters for common hexoses and pentoses are apparently not down-regulated under heterotrophic conditions. Uptake of deoxysugars (FUC/Rha), however, was repressed by substrates metabolized via gluco-, galacto-, glycero-, or hexokinase, whereas substrates phosphorylated by xylulokinase had no effect. This indicates that several sugar kinases play a key role in sugar sensing. In contrast, polyol transporters were repressed only by glucose or its analogs but not by other sugars. This repression does not involve the activity of kinases. Most likely this type of glucose sensing is independent of metabolism and takes place prior to or during uptake. In its natural environment, these two different sensing mechanisms would enable the alga to utilize a mixture of different substrates in a most economic way by repressing dispensible transporters.

Plasma membrane phosphatidylinositol 4,5-bisphosphate levels decrease with time in culture.

During the stationary phase of growth, after 7 to 12 d in culture, the levels of phosphatidylinositol 4,5-bisphosphate (PtdInsP(2)) decreased by 75% in plasma membranes of the red alga Galdieria sulphuraria. Concomitant with the decrease in PtdInsP(2) levels in plasma membranes, there was an increase in PtdInsP(2) in microsomes, suggesting that the levels of plasma membrane PtdInsP(2) are regulated differentially. The decline of PtdInsP(2) in plasma membranes was accompanied by a 70% decrease in the specific activity of PtdInsP kinase and by reduced levels of protein cross-reacting with antisera against a conserved PtdInsP kinase domain. Upon osmotic stimulation, the loss of PtdInsP(2)from the plasma membrane increased from 10% in 7-d-old cells to 60% in 12-d-old cells, although the levels of inositol 1,4,5-trisphosphate (InsP(3)) produced in whole cells were roughly equal at both times. When cells with low plasma membrane PtdInsP(2) levels were osmotically stimulated, a mild osmotic stress (12.5 mM KCl) activated PtdInsP kinase prior to InsP(3) production, whereas in cells with high plasma membrane PtdInsP(2), more severe stress (250 mM KCl) was required to induce an increase in PtdInsP kinase activity. The differential regulation of a plasma membrane signaling pool of PtdInsP(2) is discussed with regard to the implications for understanding the responsive state of cells.

The gene family coding for the light-harvesting polypeptides of Photosystem I of the red alga Galdieria sulphuraria.

Recently [Marquardt et al. (2000) Gene 255: 257-265], we isolated a gene encoding a polypeptide of the light-harvesting complex of Photosystem I (LHC I) of the red alga Galdieria sulphuraria. By screening a G. sulphuraria cDNA library with a DNA probe coding for the conserved first transmembrane helix of this protein we isolated four additional genes coding for LHC I polypeptides. The deduced preproteins had calculated molecular masses of 24.6-25.6 kDa and isoelectric points of 8.09-9.82. N-terminal sequencing of a LHC I polypeptide isolated by gel electrophoresis allowed us to determine the cleavage site of the transit peptide of one of the deduced polypeptides. The mature protein has a calculated molecular mass of 20.6 kDa and an isoelectric point of 7.76. The genes were amplified from nuclear G. sulphuraria DNA by polymerase chain reaction (PCR) using oligonucleotides annealing in the regions of the start and stop codons as primers. All genomic sequences were 80-300 base pairs longer than the PCR products obtained from the respective cDNA clones, pointing to the existence of 1-5 introns per gene. The G. sulphuraria genes form a homogeneous gene family with overall pairwise amino acid identities of 46.0-56.6%. Homology to two diatom, one cryptophytic and two higher plant light-harvesting polypeptides was lower with pairwise identities of 21.1-34.1%. Only one diatom polypeptide showed a higher degree of identity of up to -39.3%.

Intron–exon structure and gene copy number of a gene encoding for a membrane-intrinsic light-harvesting polypeptide of the red alga Galdieria sulphuraria

Genes for light-harvesting proteins ( lhc genes) of higher plants are well examined. However, little is known about the corresponding genes of algae, although this knowledge might give valuable information about the evolution of photosynthetic antennae. In the case of rhodophytes only two cDNA sequences from a single organism, Porphyridium cruentum, have been published. Here we describe an additional sequence from another species, the thermo-acidophilic red alga Galdieria sulphuraria. For the first time also a genomic sequence for a red algal lhc gene is presented. From a cDNA library of G. sulphuraria we isolated a clone containing an open reading frame for a protein of 302 amino acids with a deduced molecular mass of 33.86 kDa. It shares major structural features with eukaryotic light-harvesting polypeptides. A proposed cleavage site between transit peptide and mature protein gives rise to a transit peptide of 119 amino acids and a mature protein of 183 residues. Hydropathy analysis suggests that the mature protein consists of three transmembrane helices. Several amino acid residues supposed to bind chlorophyll a and chlorophyll b in higher plants are conserved. The protein shows up to 69% identity and 81% similarity to the Porphyridium polypeptides in the transmembrane helices 1 and 3. Using oligonucleotides annealing in the regions of the start and stop codons of the gene as primers, a DNA sequence was amplified from nuclear G. sulphuraria DNA by PCR. Compared with the cDNA clone, this sequence contains five additional intervening DNA strings of 50–74 bp length. Four of them show typical features of spliceosomal introns with GT–AG borders, and the fifth differs by starting with GC. Three of the supposed introns are located in similar positions as introns of higher plant light-harvesting proteins. Southern blotting and hybridization experiments indicate that G. sulphuraria contains at least three copies of this gene.

Characterization of a sugar/polyol uptake system in the red alga Galdieria sulphuraria

The unicellular, acido- and thermophilic red alga Galdieria sulphuraria is unique in its ability to grow heterotrophically on at least 27 different sugars and polyols. The enzymatic machinery necessary for metabolizing this variety of compounds is constitutively expressed in the alga. The uptake system, however, has to be induced. From in vivo studies we conclude that the uptake system consists of several transporters with a partly overlapping substrate specificity. These transporters can be grouped into hexose, polyol and pentose transporters. The mode of induction depends on the substrate supplied for heterotrophic growth. When autotrophic cells were supplied with hexoses, hexose and polyol transporters were induced from the onset of heterotrophic conditions. Depletion of substrate in the medium triggered the induction of other transporters. Dulcitol, as well as other polyols and pentoses, led to a co-induction of all transporters as judged from induction and competition experiments. The heterotrophic capabilities of Galdieria are intriguing, because the natural habitat of the alga contains only traces of organic carbon. The substrates for heterotrophic growth apparently originate from decaying cells releasing hydrolysed cell material. Trace amounts of glucose were sufficient for the induction of sugar uptake in Galdieria. The co-induction of transporters obviously enables the cells in endolithic mats to import a wide spectrum of organic carbon in order to survive periods of light limitation.