Relationship between biosynthesis of carotenoids and increasing complexity of photosystem I in mutant C-6D of Scenedesmus obtiquus |
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Authors: | S Römer K Humbeck H Senger |
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Institution: | (1) Fachbereich Biologie/Botanik, Philipps-Universität, Lahnberge, D-3550 Marburg, Germany |
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Abstract: | Dark-grown cells of the pigment mutant C-6D of Scenedesmus obliquus, strain D3 (Gaffron 1939), contain only chlorophyll (Chl) a and carotenoid precursors. In these cells a functioning photosystem I (PSI) of basic structure was characterised by a high PSI activity and a low Chl/P700 ratio. The reaction-center complex of PSI (CPI) was shown to exist in the dark-grown cells. These findings demonstrate that the assembly of the core complex of PSI and its function are independent of the presence of carotenoids. Upon illumination, carotenoids, Ch1 b and additional Chl a were synthesized. Newly formed -carotene was shown by pigment analysis using high-performance liquid chromatography (HPLC) to be incorporated into CPI. Parallel to this process a shift of the long-wavelength fluorescence emission of PSI from 712–714 to 718–719 nm was observed. In the later stages of chloroplast differentiation, when xanthophylls and Chl b were synthesized, a higher-molecular-weight complex of PSI (CPIa) could be isolated. Pigment analysis demonstrated that CPIa contained xanthophylls and Chl b in addition to Chl a and -carotene. This indicates the formation of a light-harvesting antenna closely associated with PSI (LHCI). The addition of an LHCI to the reaction-center complex of PSI caused an increase in the absorption cross-section of PSI as shown by action spectroscopy and in-vivo fluorescence measurements. A model demonstrating the changes in the molecular organization of PSI during light-induced carotenoid biosynthesis in mutant C-6D of Scenedesmus obliquus is presented.Abbreviations Chl
chlorophyll
- CP
chlorophyll-protein complex
- LHC
light-harvesting complex
- HPLC
high-performance liquid chromatography
- PSI, II
photosystem I, II
- PAGE
polyacrylamide gel electrophoresis
This work was supported by the Deutsche Forschungsgemeinschaft and a scholarship of the Studienstiftung des deutschen Volkes to S. Römer. We thank Ms. K. Bölte for technical assistance and Mr. H. Becker for drafting the figures. |
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Keywords: | Chloroplast differentiation Light-harvesting complex I Long wavelength antenna Photosystem I Pigment-protein complex Scenedesmus |
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