Cyanobacteria phycobilisome

In contrast to the photosynthetic eukaryotes, photoprotection in cyanobacteria is not induced by the presence of a transthylakoid ApH or the excitation pressure on PSII. Instead, intense blue-green light (400-550 nm) induces a quenching of PSII fluorescence that is reversible in minutes even in the presence of translation inhibitors (El Bissati et al. 2000). Fluorescence spectra measurements and the study of the NPQ mechanism in phycobilisome- and PSII-mutants of the cyanobacterium Synechocystis PCC6803 indicate that this mechanism involves a specific decrease of the fluorescence emission of the phycobilisomes and a decrease of the energy transfer from the phycobilisomes to the RCs (Scott et al. 2006, Wilson et al. 2006). The site of the quenching appears to be the core of the phycobilisome (Scott et al. 2006, Wilson et al. 2006, Rakhimberdieva et al. 2007b). 

Joshua, S., S. Bailey, N. H. Mann, and C. W. Mullineaux (2005). Involvement of phycobilisome diffusion in energy quenching in cyanobacteria. Plant Physiol 138(3) 1577-1585. 

Wilson, A., G. Ajlani, J. M. Verbavatz et al. (2006). A soluble carotenoid protein involved in phycobilisome-related energy dissipation in cyanobacteria. Plant Cell 18(4) 992-1007. 

FIGURE 19-43 A phycobilisome. In these highly structured assemblies found in cyanobacteria and red algae, phycobilin pigments bound to specific proteins form complexes called phycoerythrin (PE), phycocyanin (PC), and allophycocyanin (AP). The energy of photons absorbed by PE or PC is conveyed through AP (a phycocyanobilin-binding protein) to chlorophyll a of the reaction center by exciton transfer, a process discussed in the text. 

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.

Phycobiliproteins are found also in cryptophytes but, differently from cyanobacteria and red algae, they are not organized into a phycobilisome, but instead they are located in the thylakoid lumen. Unique for cryptophytes, their phycobiliproteins do not exhibit a trimeric aggregation state characteristic for cyanobacteria, but instead they are present as ai(3a2(3 heterodimers, with each a subunit having a distinct amino acid sequence. [40]... 

Carotenoids are found in all native photosynfhetic organisms. They serve a dual function, as both accessory antenna pigment and also are essential in photoprotection of photosynfhetic systems from the effects of excess light, especially in the presence of oxygen. Bilins are open-chain tetrapyrroles that are present in antenna complexes called phycobilisomes. These complexes are found in cyanobacteria and red algae. Structures of representative carotenoid pigments are shown in Figure 3. 

Accessory light-harvesting antenna systems (phycobilisomes) of cyanobacteria, red algae and of cryptomonads... 

Samsonoff W. A. and MacCoU R. (2001) Biliproteins and phycobilisomes from cyanobacteria and red algae at the extremes of habitat. Arch. Microbiol. 176, 400—405. 

Phycobilisomes were discovered more than 30 years ago by Elizabeth Gantt and Sam Contiin the outer thylakoid layer ofthe red alga Porphyridium cruentum. Phycobilisomes are present in large quantities in cyanobacteria and red algae, and may amount to as much as 50% ofthe soluble protein ofthe cell. Intact phycobilisomes can be readily dislodged intact from the membrane with a detergent such as Triton... 

Fig. 2. Electron micrographs of phycobilisomes in the red alga Rhodella violacea (A) and phycobilisomes isolated from Porphyridium cruentum (B). (C) shows a membrane model consisting of the electron-transfer complexes of PS I, PS II, the cytochrome bet complex, the ATP synthase, CFq CFi, and the phycobilisomes. (A) and (C) from MOrschel and Rhiel (1987) Phycobilisomes and thylakoids The light-harvesting system of cyanobacteria and red algae. In JR Harris and RW Horne (eds) Membranous Structure, pp 216, 248. Acad Press (A) kindly furnished by Dr. Erhard Mbrschei and (B) kindly furnished by Dr. Alexander Glazer.

As seen earlier in Fig. 2, a phycobilisome is a fan-shaped set ofsupramolecular assemblies, consisting of a triangular core with six short rods radiating outward, with each phycobilisome being attached to the thylakoid membrane through the core complex. In crude extracts of cyanobacteria, these short rods are seen in electron micrographs to have a diameter of 12 nm, as expected for stacks of hexamers, with clearly marked divisions 6 nm apart along the rod and faint divisions midway between. 

The phycobilisome core is the site of attachment to the thylakoid membrane. Hemidiscoidal phycobilisomes, shown earlier in Fig. 2, are present in both cyanobacteria and some red algae, while hemiellipsoidal phycobilisomes are found only in certain other red algae. The phycobilisome in the thylakoid-less cyanobacterium Gloeobacter violaceus is present as a bundle of six rods in the phycobilisome of Synechococcus 6301, the core (attached to the thylakoid) has only two short rods attached. 

WA Sidler (1995) Phycobilisome and phycobiliprotein structures. In DA Bryant (ed) Molecular Biology of Cyanobacteria, pp 139-216. Kluwer... 

Geiselmann, H., Houmard, J., and Schoefs, B. 2004. Regulation of phycobilisome biosynthesis and degradation in Cyanobacteria. In Handbook of Photosynthesis (M. Pessarakli, ed.), 2nd Ed. Marcel Dekker, New York. 

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