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Analysis of the Molecular Organization of Photosystem I During Light‐Dependent Chloroplast Differentiation in Mutant C‐6D of Scenedesmus obliquus *
Author(s) -
Römer S.,
Humbeck K.,
Senger H.
Publication year - 1990
Publication title -
botanica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.871
H-Index - 87
eISSN - 1438-8677
pISSN - 0932-8629
DOI - 10.1111/j.1438-8677.1990.tb00142.x
Subject(s) - photosynthetic reaction centre , photosystem , fluorescence , pigment , chlorophyll , chloroplast , photosystem ii , photochemistry , p700 , chemistry , chlorophyll a , bacteriochlorophyll , photosynthesis , photosystem i , biology , electron transfer , botany , biochemistry , optics , physics , organic chemistry , gene
Cells of pigment mutant C‐6D of the green alga Scenedesmus obliquus synthesize only Chl a and precursors of carotenoids during heterotrophic growth in the dark. These cells exhibit high PSI‐activity per Chl and a low Chl/P700‐ratio. After transfer to light, Chl a, Chl b and carotenoids are formed with different kinetics. Analysis of chlorophyll fluorescence emission and excitation spectra revealed a sevenfold increase in the amount of the long wavelength antenna of PSI (720 nm) resulting in an increase in the absorption cross section of PSI during illumination. The underlying changes in molecular organization of PSI were investigated by sucrose density centrifugation of solubilized thylakoids after digitonin treatment and subsequent identification of the components by gel electrophoresis, HPLC and fluorescence. In dark grown cells one blue‐green band (0‐II) could be resolved. This band contained only Chl a and the reaction center complex of PSI, CPI. After 24 hours of illumination three pigmented zones and a small amount of free pigment were observed. One of the zones (24‐I) was identified as a light‐harvesting fraction containing the pigment‐protein complexes LHCP 1 and LHCP 3 . In the second fraction (24‐II) the reaction center complexes of PSI and PSII were found. The highest molecular weight fraction (24‐III) was enriched in PSI‐complexes of higher molecular weight and contained a high amount of long wavelength fluorescence antenna (720 nm) attributed to PSI. In contrast to band 24‐II which contained a high percentage of β‐carotene and a high Chl a/b‐ratio, the Chl a/b‐ratio of fraction 24‐III was lower and the xanthophyll content increased. Our data demonstrate an increase in the PSI‐unit size during chloroplast development in mutant C‐6D of Scenedesmus obliquus . Dark‐grown cultures have small functional PSI‐units composed of the chlorophylls involved in charge separation and the core antenna. This unit contains only Chl a and no carotenoids. After transfer to light Chl b and carotenoids are formed. Simultaneously with the appearance of carotenoids and Chl b, PSI‐complexes of higher molecular weight are synthesized indicating the addition of a LHC to the reaction center complex of PSI.