Laboratory of Plant Physiology & Metabolism
(Shiraiwa & Suzuki Lab.)

1-1-1, Tenno-dai, Tsukuba, Ibaraki, 305-0821, JAPAN
Laboratory of Plant Physiology and Metabolism,
Graduate School of Life and Environmental Sciences,
Unversity of Tsukuba

Tel&Fax (029)853-4908

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Poster ポスター (A0)

Pamphlet パンフレット (A4)

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卒研生・大学院生・研究マインド研究生(生物学類)を募集しています. まずはどなたか教員にご連絡後, ラボ見学に来てみてください.

Research 研究内容


■ Coccolithophore, Emilanina huxleyi

Carbon Assimilation
Biomineralization
Alkenone Biosynthesis
Selenium Utilization

■ Cyanobacterium, Synechocystis sp. PCC 6803

Two-component System
○ Photosystems
○ Fatty-acid Biosynthesis

■ Green alga, Chlamydomonas reinhardtii

High-CO2 assimilation

■ Green alga, Botryococcus braunii

Biofuel production

■ Others

Trace Element Utilization

○ Carbon Assimilation

Eukaryotic phototrophs acquired their chloroplasts through multiple endosymbiotic events and then distribute across diverse lineages. According to wide phylogenetic variation, eukaryotic algae are expected to have diversity in their primary carbon metabolisms. Despite the extensive studies on the photosynthetic carbon metabolisms in the primary endosymbiotic algae such as green algae, studies on ecologically important marine phytoplanktons, which are mostly the secondary endosymbiotic algae, are quite limited. Coccolithophorids (Haptophyta) are unicellular calcifying algae that are widely distributed in the world's oceans. They are evolved through the secondary endosymbiosis of red algae.
The representative coccolithophorid, Emiliania huxleyi, frequently forms huge blooms that cover > 100,000 km2 of the ocean surface and gives significant impact on the global carbon cycle. Despite their ecological and evolutional importance, the primary carbon metabolism in E. huxleyi is poorly characterized, especially at molecular level. Though E. huxleyi accumulates unique photosynthetic products such as beta-glucan, acidic polysaccharides, and alkenones, the biosynthetic pathway of these compounds have not been understood. Our aim is to identify the pathway and enzymes involved in the photosynthetic carbon metabolism of E. huxleyi by the combination of transcriptomics, proteomics, and enzymological analysis.


○ Biomineralization

Biomineralizaion refers to the process by which organism precipitate inorganic minerals. This phenomenon is widespread in the biological world, and occurs in bacteria, plants and animals. Crystal structure of minerals formed by organic process is different from that by inorganic process. Changes of the character between organic minerals and inorganic minerals are also observed. Minerals produced by biomineralizaion are diverse, such as calcium phosphate in human bones and teeth, calcium carbonate in animal shells and silicate in plant opals. It is presumed that calcium carbonate is mainly produced by marine organisms.
Coccolithophorids are a group of marine haptophyte algae that form calcium carbonate crystals in the cell. The crystal with a fine structure, named coccolith, in the coccolith vesicle that was deprived from Golgi body, then transported onto the cell surface. In our laboratory, a coccolithophorid, Emiliania huxleyi, is mainly utilized for our experiments.
Formational process of coccolith is predicted to include various mechanisms, such as formation of structure of specific membranes, transport of large amount of calcium ions, and so on. However, almost mechanisms are until unknown. Since an acid polysaccharide called coccolith polysaccharide (CP) is obtained by melting coccolith, it is suggested that this CP is involved in the morphogenesis of coccolith. However, almost synthetic system of this CP is still unknown.
We attempt to elucidate the regulatory mechanism of coccolith formation by analyses of biopolymers that are involved in the formational process of coccolith.


○ Alkenone Biosynthesis

Long-chain alkenones (C37-C39) are known as specific lipid biomarker of species of haptophycean algae. The double-bound configuration of fatty acid ester in such membrane-bound lipids as phospholipids and galactolipid is generally cis, while that of alkenones has been found to be trans.
Long-chain alkenones have been used for determining the paleotemperature in geochemical and geophysical sciences, since the number of these double bounds in the molecules is produced in response to the prevailing temperature during their growing.
It is still unknown why haptophycean algae synthesize alkenones and how haptophycean algae synthesize and/or degrade alkenones. Since only five species of haptophyceae including Emiliania huxleyi is known to accumulate alkenones, it is thought that the ability to synthesize alkenones is newly acquired in the evolutionary process of haptophyceae. In our laboratory, we aim to elucidate physiological function and metabolic system of alkenones in haptophycean algae.


○ Selenium Utilization

Emiliania huxleyi, an unicellular calcifying alga, is an abundant coccolithophorid in the ocean. E. huxleyi is known to fix a large amount of carbon and produce a huge biomass during their blooms. In previous studies, we revealed that selenium (Se) is an essential element for their growth. Se is very similar element with S in physicochemical property, and is an essential element for various organisms such as mammals including human and bacteria. The essential nutritional function of Se is due to the action of selenoproteins containing Se in the form of selenocysteine (Sec or SeCys); such selenoproteins play essential roles in maintaining cell viability. Although Se is an essential element for many organisms, it is also toxic at higher concentrations. A few land plants have been found to accumulate Se up to a thousand-fold in the plant body. However, land plants don't require Se for their growth. So, it is thought to reduce the toxic effects of Se by metabolizing inorganic Se ions to non-toxic organic compounds. Therefore, there is very little information on Se metabolism and selenoproteins in photosynthetic organisms that show requirement for Se.
We utilize E. huxleyi for our experiments in order to elucidate physiological functions of Se in photosynthetic organisms. Further, it is expected to obtain important information to elucidate species-specific strategy for Se-utilization by the analyses of Se-metabolic mechanism in E. huxleyi.


○ Two-component System

Cyanobacterium is a group of photosynthetic bacteria universally observed under various environments, i.e. fresh and marine water, low/high temperature, high salinity, high osmotic pressure and drought stress condition. They may possess the acclimation mechanisms to several environmental conditions. Please imagine that, we can easily immigrate to different places when environmental change occurred, i.e. relax in shaded area to escape from strong sunshine. However, it is impossible for land plants or microorganism to look for and move to comfort places. They have developed the acclimation mechanism to survive even if they were exposed to "inconvenient condition".
The primary event of the acclimation is perception of "what is changed?" by cells. How do organisms realize environmental changes and reflect in cell homeostasis.
There have been studied about signaling process of bacteria (i.e. Escherichia and Bacillus) by molecular-based analysis and it revealed the existence of "two-component system", which is responsible for signal perception and its transduction. This system is found in bacteria, fungi and land plant, and is known to be composed of two proteins: histidine kinase and response regulator. Once Histidine kinase perceives the environmental change by a signal-input domain located at its N-terminal region, it changes phosphorylation state of the histidine residue of kinase domain located at its C-terminal region. In general, response regulator, that is a transcriptional factor containing DNA-binding motif at the C-terminal portion, receives phosphate group from the cognate histidine kinase on its specific phosphorylation state at aspartate residue and alters ability to activate or deactivate RNA polymerase to induce or repress the expression of target genes. In our laboratory, we are focusing on two component systems in Synechocystis sp. PCC 6803, which is a photosynthetic cyanobacterium used as a model organism for photosynthesis studies.
Genomic sequence of Synechocystis revealed that this organism possesses 44 genes for histidine kinases and 42 genes for response regulators in the chromosome and each 3 genes for histidine kinases and response regulators in the plasmids. We established the Synechocystis mutant library inactivated each gene for histidine kinase and performed their characterization by transcriptomic analysis and revealed manifold signals perceived by two component system: low and high temperature stress, salinity stress, high osmotic pressure, phosphate deficiency, oxidative stress, high light intensity and so on.
We are studying about molecular mechanism of signal perception by histidine kinase. Interestingly, we investigated unique histidine kinases in Synechocystis, i.e. a hitidine kinase which is responsible for several kinds of stress perception, a histidine kinase which orthologue gene is encoded in chloroplast genome of eukaryote microalgae. And we are also interested in small cofactor proteins involved in signaling activity of histidine kinase.


○ High-CO2 assimilation

Chlamydomonas reinhardtii, the freshwater unicellular green alga, called 'green yeast' can acclimate various environmental changes. By many studies on mechanisms of response to these environmental stress, unique physiological mechanisms are revealed. Since the alga is a model system for studying photosynthesis, detailed studies on acclimation to a change of external CO2 concentration have been performed.
In previous studies, we isolated a novel periplasmic protein with a molecular mass of 43 kD and named it H43 in cells grown under high-CO2 conditions in autotrophic culture. Further, we revealed that Fe-deficiency is also a signal for the H43 induction, independent of high-CO2 signal. Since both CO2 and Fe availability are involved in photosynthesis, it is very interesting to elucidate regulatory mechanism of H43 gene expression.
In our laboratory, H43 is therefore utilized as a tool for reporting gene expression under high-CO2 and Fe-deficient conditions. We aim to elucidate mechanisms of response to high CO2 and Fe-deficiency of Chlamydomonas by identification of transcriptional cis-elements of H43 gene, trans-acting factors binding H43 gene, and so on.


○ Biofuel production

Green alga, Botryococcus is a unique microorganism because they can accumulate high content of diesel oil-like hydrocarbons. In contrast to bioethanol production from crops, hydrocarbons from Botryococcus is expected to be a new renewable energy because of its low environmental load and low effect on our food supply situation. However, for its commercial production, there remain a lot of problems yet. It required manifold basic knowledges, i.e. metabolic pathway to synthesize hydrocarbons and culture condition which induce hydrocarbon production. In our laboratory, we are studying physiologically on Botryococcus biofuel production.
In order to drive hydrocarbon production by Botryococcus, it is the first key to investigate the optimal condition for Botryococcus growth. In photosynthetic organisms, all organic compounds including hydrocarbons are synthesized from carbons which is assimilated by photosynthesis. Therefore, it is expected that the induction of photosynthetic activity to uptake more carbons into cells causes the more hydrocarbons production.
Nitrogen starvation is well-known to be a condition which induces the production of carbon storage compounds (triacylglyceride, polysaccharides). Because under nitrogen deficiency condition, the carbon flux into amino-acids biosynthesis was stopped (amino-acids contain nitrogen) and the excess carbon flow is switched into biosynthesis of carbon storage compound (lipids do not contain nitrogen). To stimulate the photosynthesis and to regulate the carbon flux into hydrocarbon biosynthesis should cause higher production of hydrocarbons.


○ Trace Element Utilization

1. Iodine accumulation and utilization by microalgae.

Although the land of Japan is poor producer of underground resources (crude oil, coal or gas), when mentioning "iodine", Japan can provide iodine at about half of world demand. Almost all iodine in Japan is produced in Chiba prefecture. Iodine extracted from Chiba is contained together with natural gas in brine water: concentrated marine water found in underground. It is expected that present iodine have been produced biologically in ancient time.
Underground iodine is thought to be produced via following scheme. Iodide ion or iodized product was absorbed into microalgae -> Sedimentation -> The plate movement below the land of Japan. However, nobody showed the biological evidence for biological process of iodine accumulation. We isolated and characterized the microalga which can highly accumulate iodide ion in the cell. It indicated the physiological function of iodide in microalgae and the involvement of microalgae into global iodide cycle.

2. Isolation of gene involved in cadmium accumulation

Cadmium (Cd) is one of the toxic elements and Cd contamination into environment is severely restricted. Therefore, elimination of Cd from contaminated soil or chemical wastewater is very important process for the conservation of the environment. In our laboratory, for the technology development of biological remediation of Cd, we are trying to isolate and characterize the gene which is responsible for Cd accumulation, by using meta-genomic library isolated from natural environment.