Chlorosomes reaction center-antenna complex, model

Chapter 8 The Green Bacteria I. The Light-Harvesting Complex, the Chlorosomes 149 I. Models for the Reaction Center-Antenna Complex of Green Bacteria... 

Fig. 3 Schematic model of light-harvesting compartments in photosynthetic organisms and their position with respect to the membrane and the reaction centers. RC1(2) Photosystem I(II) reaction centre. Peripheral membrane antennas Chlorosome/FMO in green sulfur and nonsulfur bacteria, phycobilisome (PBS) in cyanobacteria and rhodophytes and peridinin-chlorophyll proteins (PCP) in dyno-phytes. Integral membrane accessory antennas LH2 in purple bacteria, LHC family in all eukaryotes. Integral membrane core antennas B808-867 complex in green nonsulfur bacteria, LH1 in purple bacteria, CP43/CP47 (not shown) in cyanobacteria and all eukaryotes.

We now look at a recent model for the structure of the antenna complex and its relationship to the reaction center in the cytoplasmic membrane, as shown in some detail in Fig. 2. The size of chlorosomes ranges from 50 to 100 nm in width and -100 to 300 nm in length. The outer layer, or envelope, of the chlorosome is a galactolipid monolayer in which polypeptides of 18- and 11 -kDa molecular mass are embedded. Inside the lipid envelope are rodlike structures, the so-called rod elements, as previously noted, each of which is about 5-10 nm in diameter and run nearly the full length of the chlorosome. The rod elements were previously thought to contain BChl c-protein complexes, but recent studies have shown that the BChl c molecules are present simply as self-aggregated oligomers. 

FIGURE 1 Schematic models of chlorosomes from the green gliding bacteria, e.g. Chloroflexus aurantiacus (A) and the green sulfur bacteria, e.g. Chlorobium vibrioforme (B). The numbers refer to wavelength maxima of antenna bacteriochlorophyll complexes. The reaction centers are of the "quinone type in A, with a QA Qb acceptor complex and the Fe-S type in B, with Fe-S proteins as early electron acceptors. The rod elements probably contain oligomers of bacteriochlorophyll c, d or e, as discussed in Ae text. 

The light-harvesting system of green bacteria consists of BChl c containing chlorosomes attached to the cell membrane by a baseplate containing BChl a. The cell membrane contains the B 808-866 antenna complex and the reaction center. We report here a comparison of the properties of chlorosome preparations obtained by different treatments. Energy transfer in these preparations was studied by picosecond absorbance recovery and steady-state fluorescence measurements. The results are used to propose a new model for the structure of chlorosomes. 

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