Dr. Yukako Hihara

Acclimation responses of photosynthetic organisms

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Elucidation of Molecular Mechanisms Regulating Acclimation Responses
-Can Photosynthesis Be Controlled by Genetic Engineering of Master Regulators? -

Cyanobacteria, an experimental material of my research, have survived in various environments  since their first appearance on earth about 3 billion years ago. Photosynthesis  is a process to convert light energy from the sun into chemical energy. Starting with photosynthesis, various essential metabolic reactions are carried out within cells. To facilitate these reactions, various regulatory (acclimation) reactions should be carried out within cells, depending on changes in environmental conditions, such as light, temperature, and nutrients. For example, under weak light conditions, the amounts of photosynthetic apparatuses   increase to enhance their photosynthetic activity, while, under strong light conditions, they decrease because absorption of excessive light energy may generate toxic reactive oxygen species. Then, how is the amount of photosynthetic apparatuses controlled? How do cyanobacteria sense the changes in light intensity? Then, what changes occur within the cells? We have conducted research to elucidate the mechanisms of signal transductions from perception of environmental changes to each acclimation response , searching  various regulatory factors involved in environmental responses. In the future, the activities of regulatory factors can be modulated  to control photosynthesis or increase stress tolerance.

Cyanobacteria, an experimental material of my research, have survived in various environments  since their first appearance on earth about 3 billion years ago. Photosynthesis  is a process to convert light energy from the sun into chemical energy. Starting with photosynthesis, various essential metabolic reactions are carried out within cells. To facilitate these reactions, various regulatory (acclimation) reactions should be carried out within cells, depending on changes in environmental conditions, such as light, temperature, and nutrients. For example, under weak light conditions, the amounts of photosynthetic apparatuses   increase to enhance their photosynthetic activity, while, under strong light conditions, they decrease because absorption of excessive light energy may generate toxic reactive oxygen species. Then, how is the amount of photosynthetic apparatuses controlled? How do cyanobacteria sense the changes in light intensity? Then, what changes occur within the cells? We have conducted research to elucidate the mechanisms of signal transductions from perception of environmental changes to each acclimation response , searching  various regulatory factors involved in environmental responses. In the future, the activities of regulatory factors can be modulated  to control photosynthesis or increase stress tolerance.


Process
  1. Starting point
    We want to discover regulatory factors involved in the acclimation responses of cyanobacteria!
  2. Discovery of regulatory factors
    Two methods are employed to discover regulatory factors.
    a.    A mutant strain with abnormal acclimation response is isolated to identify a causative gene mutation. (Forward     genetics)
    b.    Potential “genes of regulatory factors (?)” are selected from genomic DNA sequence information. Mutants deficient in these genes are created to examine whether or not abnormal acclimation responses occur. (Reverse genetics)
  3. Main subjects of basic research
    The functions of identified regulatory factors are investigated in detail. Under what conditions and how do they function? Are there any other regulatory factors that function together? Gene-deficient strains are analyzed. Regulatory factors are purified to investigate their properties.
  4. Bridge to applied research
    How are photosynthesis and metabolic activities altered by increasing or decreasing theamounts of regulatory factors?

Profile

Yukako Hihara

Associate Professor, Area of Biochemistry and Molecular Biology, Division of Life Science, Graduate School of Science and Engineering 

Born in Tokyo

 

March 1993

Graduation from Department of Biological Sciences, Faculty of Science, The University of Tokyo

March 1995

Completion of the Master Course, Department of Biological Sciences, Graduate School of Science, The University of Tokyo

March 1998

Completion of the Doctor Course, Department of Biological Sciences, Graduate School of Science, The University of Tokyo
Received PhD. degree in Science

Between April 1997 and March 2000

Full-time Instructor,Faculty of Engineering, Saitama University

April 2000

Assistant professor, Department of Biochemistry and Molecular Biology, Faculty of Science, Saitama University

January 2009

Associate professor, Graduate School of Science and Engineering, Saitama University

April 2011~March 2014

Researcher of the PRESTO, Japan Science and Technology Agency (JST)

Hobby: Playing the viola in an orchestra or chamber music conce


Starting Point for Our Research

 

DNA Microarray Analysis Under Weak and Strong Light Conditions

Genomic DNA contains a genetic blueprint for protein expression. The blueprint generates gene copies, i.e., single-stranded mRNAs. This process is called transcription. Subsequently, the nucleotide sequences of the mRNAs are translated. Specifically, amino acids are assembled into proteins. Not all genes are always transcribed and translated. Only genes required under certain conditions are transcribed and translated (i.e., expressed) to produce proteins. Also, in cyanobacteria, different genes are expressed depending on cellular conditions. Before getting a position at Saitama University, I had investigated the gene expressions of cyanobacteria under weak and strong light conditions using a technique called DNA microarray analysis.

 


Gene expression levels under strong light conditions

 

The above figure shows DNA microarray analysis after 15-minute strong light exposure. Each dot indicates a single cyanobacterial gene. The horizontal and vertical axes show expression levels under weak and strong light conditions, respectively. The obtained results demonstrate that the expression levels of many genes are regulated by light intensity changes and that the changes in the gene expressions lead to various acclimation responses under strong light conditions. These findings serve as a starting point for our research at Saitama University

 


Regulatory Mechanism for Gene Expressions in Response to Light Intensity Changes

 

 

Fig. 2. Mechanism for transcription factor PedR to function depending on photosynthetic activity

We isolated a transcriptional regulator, PedR, from the cyanobacterium Synechocystis sp. PCC 6803. PedR functions depending on photosynthetic activity. PedR is an active form and activates or represses the transcriptions of several genes under weak light conditions, where photosynthetic activity is low. When photosynthetic activity is enhanced by strong light exposure and  reducing equivalents from the photosynthetic electron transport chain are transmitted  to a regulatory protein called thioredoxin, the conformational change and inactivation of PedR occur immediately via interaction with thioredoxin. Subsequently, PedR gradually return to its active form, regaining transcriptional regulatory activity under prolonged strong light conditions.

 


Large-sized Cyanobacterial Mutant that Accumulates a Large Amount of Glycogen

Deleting Sll0822, a transcription factor of the cyanobacterium Synechocystis sp. PCC 6803, increased its cell size with a large amount of glycogen (storage polysaccharide) accumulated (small black particles in the cells). This property, i.e., “large-sized container rich in a substance,” is advantageous in producing a useful substance. Currently, we are trying to modify its metabolism by deleting or introducing different genes to convert highly-accumulated glycogen into fatty acids and oils.

 

Fig. 3. Transmission electron micrographs of wild-type (WT) and Sll0822-deficient (Δsll0822) strains (photographed by Professor Yasuko Kaneko of the Faculty of Education)
The arrow indicates a glycogen particle accumulated in the cells.


Cyanobacteria

Cyanobacteria is a phylum of bacteria that obtains energy for living through photosynthesis in the same mechanism as plants. Cyanobacteria inhabit various environments throughout the earth, such as ponds, lakes, sea, and lands, and their morphology is significantly diverse.

Some cyanobacterial species withstand harsh environmental conditions, such as cold and heat and high salt concentrations. Some cyanobacterial species are good research materials in that they can be easily cultured and genetically engineered. The entire genomic DNA sequences of many cyanobacterial species have been decoded. The genomic sequences provide much information regarding what kinds of genes organisms have and how they carry out metabolic reactions. We are fully utilizing the information in our research.

Cyanobacteria have played several important roles in the history of biological evolution. Cyanobacteria first appeared on earth about 3 billion years ago. Cyanobacteria actively carried out photosynthesis, absorbing carbon dioxide and releasing oxygen. Since the appearance of cyanobacteria, carbon dioxide in the atmosphere has decreased, while oxygen has rapidly increased. We thank to cyanobacterial photosynthesis for the survival of organisms, including human beings, on earth by breathing.