Phycobilisome particles

The thylakoid areas of cyanobacteria are characterized by broad lanes of densely packed protoplasmic (PF) particles of 7.0 - 9.0 nm. On the complementary fracture face, rows of well aligned exoplasmic (EF) particles of the 10 nm size class fit in the particle free grooves between the PF-particle lanes (3) The minimum centre to centre distance of these EF-particle rows is 45 nm or more and is thus in register with the spacing of phycobilisome rows (4) ... 

From our results we propose that the 10 nm EF-particles correspond to PSII-complexes. In cyanobacteria and red algae with hemi-discoidal phycobilisomes, these PSII complexes are aggregated to dimers, which bind one phycobilisome on top. Each 10 nm particle represents a PSII-complex containing at least the reaction centre complex (RC), the water splitting system, the Chl-antennae with apoproteins of 41 and 47 kDa. 

The arrangement of the EF-particles of red algae containing hemi-ellipsoidal phycobilisomes is similar. However, the EF-particle rows are loosened into packages of particles which are separated normally by... 

The concentration of PSII centres was determined by flash light induced oxygen evolution (7) and correlated with the amount of AP. A Chi to PSII ratio of 120 Chi a/PSII was determined. The molar ratio of PSII to AP was 1 5 Hemi-ellipsoidal phycobilisomes may contain 20-24 moles of AP. Thus the molar ratio of PSII to AP is 4 20 and consequently a tetrameric PSII particle package binds one hemi-ellipsoidal phycobilisome. It is supposed that the aggregation of PSII to dimers and tetra-mers is induced by the phycobilisome structures of the cores. Depending on the type of phycobilisome, different aggregation patterns are realized. 

Fig, 1.a) Freeze-fractured thylakoids of Porphyridium cruentum. Arrowheads indicate EF-particle clusters. Particle clusters are resolved into four 10 nm particles (arrows). Bar 100 nm. b,c) Phycobilisomes in profile from the side. The central furrow is marked. Bars 200 nm. 

It is clear that the ultrastructure of the phycobilisomes of Rhodosorus is markedly different to any previously described. They seem also to have a close relationship with the particle arrangement on both PF and EF faces of the thylakoid membrane. 

FIGURE 2. a) Polypeptide profiles of P. laminosum membranes (MF), PSI particles (PSI), isolated phycobilisomes (PB), PSII-enriched LDAO supernatant (PSII Snt), POPS and Fractions 1-5 from the sucrose density gradient, using the SDS-PAGE buffer system of Chua (1980). Chlorophyll loadings per track were 5 hg (MF) / 10 ig (PSI) or 2 ig (PSII Snt and gradient fractions). b) Polypeptide profile of POPS (3 ig Chi) using the buffer system of Laemmli (1970). standards (St) were BSA... 

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