National Institute for Basic Biology


Hideo Mohri
Associate Professor:
Shigeru Itoh
Research Associates:
Mamoru Mimuro
Katsunori Aizawa (on leave)
Mika Tokumoto

Evolution of photoysnthesis and the mechanism of electron transfer

Shigeru Itoh

We study the evolution of photosyntheis focusing on the mechanism of solar energy conversion. Anoxygenic photosynthesis of bacteria seems to have evolved in the Precambrian Earth just after the evolution of life. Plant-type oxygen-evolving photosynthesis, then, was established in cyanobacteria 3500-2700 million years (Ma) ago and increased atmospheric oxygen, judging from the microfossils in Stromatolites and the oxidized iron sediments. Symbiosis of cyanobacteria inside larger cells seems to have produced first plant about 2000 Ma ago.

What can we study to explore the past? We take three approaches (1) Survey of electron transfer mechanisms in photosyntheic reaction center (RC) pigment-protein complexes. We replaced quinones and chlorophylls inside RC to see what happens. We use the ps-ns laser and ESR spectroscopies at 4-300 K in the quinone-replaced RCs and found that both the plant and bacterial RCs are highly optimized (Iwaki et al, Itoh et al. and Baba et al). The result that was the first evidence of the energy-gap-dependent ultra-rapid electron tunneling in plant, also teaches us the design of prototype RC. (2) Site-directed mutagenesis to construct a prototype of electron transfer protein. We modified a ferredoxin that functions in N2-fixation and found the conserved motif to be indispensable for the required extremely strong reducing power (Saeki et al). (3) Survey of new photosynthesis. We performed comparative studies of photosynthesis in purple, green sulfur, hellio-, cyano-bacteria and plants. Our topic in 1966 was the discovery of bacteria that undergo photosynthesis with Zn-bacteriochlorophyll (Wakao et al). The finding was a surprise by itself, and also questions for the reason of ubiquitous use of Mg-chlorophylls in ever known plant and bacterial photosynthesis. We are now studying another organism that undergoes oxygen-evolving photosynthesis with a completely new type of chlorophyll.

Fig. 1.
Space-filling (lower) and wire-frame (upper) models of R. capsulata ferredoxin (FdxN). Pink-colored moiety in native FdxN (left) was altered (right) by site-directed mutagenesis. The mutation exposed the Fe-S cluster (yellow and red) and decreased its reducing power.

Primary processes of photosynthesis were investigated from various points of view; phylogeny or systematics to ultrafast spectroscopy

Mamoru Mimuro

The origin of oxygenic photosynthetic organisms is now hypothesized to be symbiosis of two different types of photosynthetic bacteria. However, it is very hard to reproduce process(es) of symbiosis in a laboratory. We started to study this process by using marine phytoplankton, dinoflagellates. We think that the first step of symbiosis is recognition of counterpart for symbiosis, thus we tried to find chemical compound(s) responsible for recognition. We found that some dinoflagellates contain lectin and some showed the lectin-receptor activity (Hori et al.). These compounds might be included in the recognition process.

Chl dimer structure is found both in reaction centers and in antenna. We have already shown that this dimerization induced a systematic decrease in an intensity of magnetic circular dichroism (MCD). We extensively analyzed this phenomenon on bacterial reaction center, green plant reaction center and also bacterial antenna (Kobayashi et al.). Furthermore, we applied this method to artificially formed chlorophyll a epimer (so called Chl a'). The MCD intensity was also half of that of monomer, indicating that decrease in the MCD intensity is universal phenomenon (Oba et al.). Spectroscopic properties of Chl a' dimer indicated a stacked face-to-face dimer structure.

Excited state dynamics of carotenoids were analyzed by the femto-second up-conversion method. Relaxation processes from the second excited singlet state (S2 state) were shown for the first time to follow the energy gap law of internal conversion (Mimuro et al., J. Am. Chem. Soc. in press). We applied this method to the peridinin-chlorophyll-complex (PCP) isolated from a dinoflagellate, and determined the energy transfer pathway from peridinin to Chl a' from the S1 state of peridinin to the S1 state of Chl a (Akimoto et al.). This is unique in the transfer pathway in algal photosynthesis. We proposed that this transfer pathway is realized by a unique chemical structure of peridinin.

Molecular structure of antenna system in a green photosynthetic bacterium, Chloroflexus aurantiacus was proposed; an array of dimer units which are arranged in a spoke-like manner in a rod structure of chlorosomes (Mimuro et al.).

Role of proteasome in meiotic cell division

Mika Tokumoto

All eukaryotic cells, from yeast to human, contain large protease complexes called proteasomes, of which the 20S and 26S proteasomes are the two major types. Recent evidence indicates that proteolysis may play an important role in the regulation of the meiotic and mitotic cell cycles. Inhibitor studies suggest that proteasomes are involved in meiotic maturation of animal oocytes. To investigate the roles of proteasomes in meiotic maturation polyclonal antisera against 20S proteasomes was used to examine changes of components of proteasome during oocyte maturation and early development of Xenopus laevis. Although no significant changes in the proteins common to 20S and 26S proteasomes were observed during oocyte maturation and early development, the amount of a unique 48 kDa protein (p48) component of the 26S proteasome decreased during oocyte maturation to the low levels seen in unfertilized eggs. p48 levels remained low in fertilized eggs until after the midblastula transition. These results show that at least one component of 26S proteasomes changes during oocyte maturation and early development, and suggest that alterations in proteasome function may be important for the regulation of developmental events, such as the rapid cell cycles, of the early embryo. Further molecular characteristics of p48 will be required to determine the function of p48.

Publication List:
Akimoto, S. (Hokkaido Univ.), Takaichi, S. (Nippon Medical School), Ogata, T. (Kitasato Univ.), Nishimura, Y. (Hokkaido Univ.), Yamazaki, I. (Hokkaido Univ.) and M. Mimuro (1996) Excitation energy transfer in carotenoid-chlorophyll protein complexes probed by femtosecond fluorescence decays. Chem. Phy. Lett., 260, 147-152.
Baba, K., Itoh, S., Hastings, G. and Hoshina, S. (1996) Photoinhibition of photosystem I electron transfer activity in isolated photosystem I preparations with different chlorophyll contents. Photosynt. Research 47, 121-130.
Hori, K. (Hiroshima Univ.), Ogata, T. (Kitasato Univ.), Kamiya, H. (Kitasato Univ.) and M. Mimuro (1996) A preliminary evidence that lectin-sugar interaction exists in the initial recognition process in symbiosis involving dinoflagellates. J. Phycol., 32, 783-790.
Ishijima, S., Kubo-Irie, M., Mohri, H. and Hamaguchi, Y. (1996) Bidirectional sliding of doublet microtubules of sea urchin sperm axonemes. J. Muscle Res. Cell Motil. 17, 287-288.
Ishijima, S., Kubo-Irie, M., Mohri, H. and Hamaguchi, Y. (1996) Calcium-dependent bidirectional power stroke of the dynein arms in sea urchin sperm axonemes. J. Cell Sci. 109, 2833-2842.
Itoh, S., Iwaki, M., Tomo, T. and Satoh, K. (1996) Dibromo-thyomoquinone (DBMIB) replaces the function of QA at 77 K in the Isolated Photosystem II reaction center (D1-D2- cytochrome b559) complex: Difference spectrum of P680+(DBMIB-) state. Plant and Cell Physiology 37, 833-839.
Iwaki, M., Kumazaki, S., Yoshihara, K., Erabi, T. and Itoh, S. (1996) Delta G0 dependence of the electron transfer rate in photosynthetic reaction center of plant photosystem I: Natural optimization of reaction between chlorophyll a (A0) and quinone. J. Phys. Chem. 100, 10802-10809.
Kawai, H. (Kobe Univ.), Nakamura, S. (Toyama Univ.), Mimuro, M., Furuya, M. (Hitachi) and Watanabe, M. (NIBB, Large spectrograph) (1996) Microspectrofluorometry of autofluorescent flagellum in phototactic brown algal zoids. Protoplasma, 191, 172-177.
Kobayashi, M. (Tohoku Univ.), Wang, Z.-Y. (Tohoku Univ.), Yoza, K. (Tohoku Univ.), Umetsu, M. (Tohoku Univ.), Konami, H. (Tohoku Univ.), Mimuro, M. and Nozawa, T. (Tohoku Univ.) (1996) Molecular structures and optical properties of aggregated forms of chlorophylls analyzed by means of magnetic circular dichroism. Spectrochim. Acta, 51A, 585-598.
Mimuro, M., Nishimura, Y. (Hokkaido Univ.), Yamazaki, I. (Hokkaido Univ.) Kobayashi, M. (Tohoku Univ.), Wang, Z.-Y. (Tohoku Univ.), Nozawa, T. (Tohoku Univ.), Shimada, K. (Tokyo Metropol. Univ.) and Matsuura, K. (Tokyo Metropol. Univ.) (1996) Energy transfer processes in chlorosomes of a green photosynthetic bacterirum, Chloroflexus aurantiacus. Photosynthe. Res., 48, 263-270.
Oba, T. (Univ. Tokyo), Watanabe, T. (Univ. Tokyo), M. Mimuro, Kobayashi, M. (Tsukuba Univ.) and Yoshida, S. (Univ. Tokyo) (1996) Aggregation of chlorophyll a' in aqueous methanol. Photochem. Photobiol., 63, 639-648.
Ogawa, K. and Mohri, H. (1996) A dynein motor superfamily. Cell Struct. Funct. 21, 343-349.
Ogawa, K., Takai, H., Ogiwara, A., Yokota, E., Shimizu, T., Inaba, K. and Mohri, H. (1996) Is outer arm dynein intermediate chain 1 multifunctional? Mol. Biol. Cell 7, 1895-1907.
Saeki, K., Tokuda, K.-I., Fukuyama, K., Matsubara, H., Nadamame, K., Go, M. and Itoh, S. (1996) Site-specific mutagenesis of Rhodobacter capsulatus ferredoxin I, FdxN, that functions in nitrogen fixation. J. Biological Chemistry 271, 31399-31406 (1996).
Wakao, N., Yokoi, N., Isoyama, N., Hiraishi, A., Shimada, K., Kobayashi, M., Kise, H., Takaichi, M., Iwaki, M., Itoh, S. and Sakurai, Y. (1996) Discovery of natural photosynthesis using Zn-containing bacteriochlorophyll in an aerobic bacterium Acidophilium rublum. Plant and Cell Physiology 37, 889-893.
Last Modified: 12:00, June 27, 1997