PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

Characterization of Photosystem I complex purified from a diatom, Chaetoceros gracilis. Yohei Ikeda^*,1, Kazuhiko Satoh¹, Yasuhiro Kashino¹, ¹ Himeji Institute of Technology, Faculty of Science, Hyogo, 678-1297, Japan

ABSTRACT- Diatoms play an important role as a primary producer in aquatic ecological system, and photosynthetically emit molecular oxygen which amounts to a quarter of annual photosynthetic oxygen evolution on earth. From a biochemical point of view, their photosynthetic system is quite interesting because diatoms, as well as other Heterokontaphyte, belong to a unique algal group of secondary symbionts and they possess unique additional photosynthetic pigments such as chlorophyll c and fucoxanthin. However, biochemical studies on their photosystems are very limited. The precise characterization of their photosytems will contribute the understanding of evolutional trail of photosystems, too. Here, we focused on the biochemical feature of photosystem I (PS I) in diatoms. Thylakoid membranes from Chaetoceros gracilis were solubilized by n-dodecyl--D-maltoside, and the subsequent sucrose density gradient centifugation and column chromatography yielded PS I complexes in a high purity. Such purified PS I complexes retained photoactive P700. Electrophoresis, western blotting and optical analyses revealed that the preparation was devoid of PS II, cytochrome b6/f nor ATPase. The purified PS I complexes contained more than 13 polypeptides ranging from 3 to 65 kDa. The N-terminal amino acid sequencing of all individual polypeptides revealed that the PS I complexes bound fucoxanthin-chlorophyll binding protein (FCP), which was also supported by spectroscopic and pigment analyses. This suggests that a specific FCP plays a role like LHC I in higher plant PS I complexes. One of the other remarkable features of this purified PS I complexes was the emission spectrum at 77 K peaking at around 710 nm, which was very short compared to reported PS I complexes from wide variety of organisms but one exception. The origin of the short wavelength fluorescence is now under investigation. On the basis of these results, we will also discuss the evolution of PS I reaction center.

KEY WORDS: Diatom, Photosystem I, Fucoxanthin-chlorophyll binding protein, Fluorescence spectrum