Acclimation responses of photosynthetic organisms

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 

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

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.