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Association of chlorophyll a/b-binding Pcb proteins with photosystems I and II in Prochlorothrix hollandica
Authors:Vladimir A Boichenko  Alexander V Pinevich
Institution:a Institute of Basic Biological Problems, Russian Academy of Sciences, Pushchino 142290, Russia
b Institute of Biology, St. Petersburg State University, St. Petersburg 198904, Russia
c A.N. Bakh Institute of Biochemistry, Russian Academy of Sciences, Moscow 117071, Russia
Abstract:Action spectra for photosystem II (PSII)-driven oxygen evolution and of photosystem I (PSI)-mediated H2 photoproduction and photoinhibition of respiration were used to determine the participation of chlorophyll (Chl) a/b-binding Pcb proteins in the functions of pigment apparatus of Prochlorothrix hollandica. Comparison of the in situ action spectra with absorption spectra of PSII and PSI complexes isolated from the cyanobacterium Synechocystis 6803 revealed a shoulder at 650 nm that indicated presence of Chl b in the both photosystems of P. hollandica. Fitting of two action spectra to absorption spectrum of the cells showed a chlorophyll ratio of 4:1 in favor of PSI. Effective antenna sizes estimated from photochemical cross-sections of the relevant photoreactions were found to be 192 ± 28 and 139 ± 15 chlorophyll molecules for the competent PSI and PSII reaction centers, respectively. The value for PSI is in a quite good agreement with previous electron microscopy data for isolated Pcb-PSI supercomplexes from P. hollandica that show a trimeric PSI core surrounded by a ring of 18 Pcb subunits. The antenna size of PSII implies that the PSII core dimers are associated with ∼ 14 Pcb light-harvesting proteins, and form the largest known Pcb-PSII supercomplexes.
Keywords:PSI and PSII  photosystems I and II  Chl  chlorophyll
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